Plate I.—Illustrating the results of Artificial Selection. Within historic times the small Wild Rock Pigeon has been evolved into the large Pouter. The figure illustrates the plasticity of a living organism.

A FIRST BOOK
IN
ORGANIC EVOLUTION

BY
D. KERFOOT SHUTE, A.B., M.D.
OPHTHALMIC SURGEON TO THE UNIVERSITY HOSPITAL (COLUMBIAN)
PROFESSOR OF ANATOMY IN THE COLUMBIAN UNIVERSITY

CHICAGO
THE OPEN COURT PUBLISHING COMPANY

London: Kegan Paul, Trench, Trübner & Co., Ltd.
Paternoster House, Charing Cross Road

1899

COPYRIGHT BY
The Open Court Publishing Co.
CHICAGO, U. S. A.
1899

All rights reserved.

THIS LITTLE BOOK IS INSCRIBED TO
DR. THEODORE GILL,
NOT ONLY IN ADMIRATION FOR HIS HIGH SCHOLARSHIP
AND EMINENT SCIENTIFIC ATTAINMENTS, BUT
ALSO IN APPRECIATION OF MANY
ACTS OF COURTESY AND
KINDNESS TO THE
AUTHOR.

PREFACE.

This little book has been written chiefly for the use of students in the Medical Department of the Columbian University. It is designed to serve only as an introduction to the study of the Development Theory, and the subject has been presented, it is hoped, in a manner that will render it interesting and easily intelligible to the general reader.

The doctrine of Evolution itself enters so largely into all those departments of knowledge that especially concern the human race, and it has so profoundly modified our ideas with regard to the origin and destiny of man, that it has attained a commanding interest and become an almost necessary ingredient in what is called a liberal education.

An overwhelming majority of Anthropologists, Zoölogists, and Botanists, and a goodly number—constantly increasing—of Christian clergymen and laymen, have been almost compelled to believe in the truth of the Evolution Theory, whether they would or not, and they cannot but realize how very widely the theory extends into almost every department of human knowledge. No one, therefore, who aspires even to a moderate degree of intellectual culture, can well afford to exclude a clear understanding of what this Doctrine of Evolution really is. It is hoped this little work will render such a conception easily attainable.

The author makes no claim for originality, unless it be in the manner of presenting the subject. He has utilized the facts collated by other observers, and sometimes quoted the exact language and expressions of well-known writers on Evolution, and has endeavored to put them together in a way that may be helpful to those who are beginning the study of the Evolution Theory.

No attempt has been made to prove the truth of the theory: this is assumed. The arguments in support of it are coextensive with our knowledge of Comparative Anatomy, Embryology, Physiology, Psychology, and many other sciences.

In the preparation of the book the author is especially indebted to his friend Prof. Theodore Gill, the eminent ichthyologist, for many valuable suggestions, and more particularly for his aid in constructing the Diagram of Development. He also desires to thank his friend Dr. A. F. A. King for his kindly assistance in preparing the manuscript for the press; and also his friend Dr. L. O. Howard, Chief of the U. S. Bureau of Entomology, for much valuable information and assistance.

D. K. Shute.

August 1, 1899, 1318 L Street, N. W., Washington, D. C.

CONTENTS.

ILLUSTRATIONS.

INTRODUCTION.

It is extremely difficult to realize the variety, wealth, and grandeur of animal and plant life as they exist on the globe to-day. Even if we endeavor to recall, in imagination, all that we have seen in streams and woodland, in ponds and rivers, in meadows, and in the air; even when we call to mind the multifarious specimens we have beheld in all the museums of natural history we have visited, and remember the most vivid descriptions that we have ever read of the wealth of tropical life;—even then we have only the faintest conception of the multitude and variety of living forms, not to mention the vanished hosts of bygone ages.

It has always been the endeavor of students of nature to reduce this great host of living creatures to order by some system of classification. First one system of classification was adopted, and then another, with the progress of Botany and Zoölogy, until eventually, before the theory of evolution was entertained by scientific men, that system was adopted which naturalists likened to a tree. In this system, those lowest organisms which cannot properly be called either animals or plants, may be represented by a short trunk. Soon this short trunk divides into two large trunks, one of which represents the animal kingdom and the other the vegetable kingdom. Each of these trunks then sends off large branches representing classes, from which smaller and more numerous branches, representing orders, are given off. From these, other branches and sub-branches are separated, representing families and genera, and finally the terminal twigs or leaves represent species.

In this tree, while there is a general advance in organization from below upwards, there are many deviations in this respect. Some leaves may be growing on different branches at the same level, which means that species belonging to widely divergent classes or orders may still possess an equal grade of organization. On the same branch there may be growing leaves at different levels. This means that one species may be more highly organized than another belonging to the same class. Not only may all living species be classified in the form of a family tree, but all the extinct hosts of species that lived in the ages of the past may be similarly classified. If all the animals that have ever lived on the globe should be represented by a tree, those existing on the earth to-day would be indicated by the topmost twigs and leaves, while the extinct forms would be represented by the trunk and main branches. (Vid. [Diagram of Development].)

The detecting of this tree-like arrangement of species in nature is the progressive work of naturalists for centuries past. At about the commencement of the present century, when it was finally detected, naturalists were unable to understand the significance of it. They did not perceive the underlying principle that accounts for the fact that groups of living forms have such natural affinities that they are arranged like a family tree.

When Darwin came upon the scene and found that his predecessors had already empirically worked out the tree-like system of classification, he convinced naturalists that the great underlying principle of this system was Heredity; and that, therefore, the grouping of living and extinct organisms in a family tree according to their natural affinities is a grouping based upon genetic affinities; and further that the ultimate meaning of classification is the tracing of lines of pedigree. This signifies that all the creatures living on the globe to-day, and all the hosts that lived in the ages of the past, are blood relations in greater or less degree, and that they have all been evolved from simple microscopic creatures that appeared on the globe at the dawn of life. All organisms, then, have undergone an evolution.

The factors of fundamental importance in the study of the theory of Evolution, or the doctrine of the Transmutations of Organisms, are Cells, Heredity and Variation, Environment, Natural Selection, and Isolation.

The evolution of an organism means its descent from preceding organisms with continuous adaptation to its Environment. Its adaptation occurs chiefly, if not entirely, through the Natural Selection of its useful Variations, and the tending of these variations to be transmitted to the offspring by the forces of Heredity. The summation of the variations, by Heredity and Isolation, leads ultimately to specific, generic, ordinal, and other differences in the descendants; leads, in other words, to the transmutations of organisms.

An organism consists of two great groups of structural units called cells,—Germ-Cells and Body-Cells,—harmoniously associated together. As far as Evolution is concerned, the most fundamental parts of an organism are the Germ-Cells.

In order that the reader may have a working knowledge of the wonderful powers of Germ-Cells and their progeny, it is vitally important that some concrete illustrations of the activities of different kinds of cells should be given.

SECTION I.
ORGANIC CELLS: THE VISIBLE UNITS OF LIFE.

ORGANIC CELLS: THE VISIBLE UNITS OF LIFE.

In the sanctuary of S. Vitale at Ravenna, in Italy, is a very interesting representation, in mosaic pictures, and over life-size, of the Emperor Justinian and his Empress Theodora, attended by a numerous suite of ladies and courtiers. The mosaics are small bits of glass, of varying pattern and color, cemented together so nicely as to form beautiful delineations of the Emperor and his attendants.

The bits of glass or mosaics that form the figures may very appropriately be called structural units.

The body of man, a bird, a lizard, an oak tree, and many other animals and plants, may usefully be compared to such mosaic figures; for, just as the mosaic figure has its structural unit, the little bit of glass or stone, called the mosaic; so the bodies of men, birds, and other creatures may be looked upon as infinitely complex figures formed of minute mosaics called cells.

The groups of minute mosaics or cells that make up the bodies of animals and plants differ profoundly from the mosaics that form the figures of the dead Emperor and his companions, inasmuch as each mosaic or cell of the animal or plant body is so small as to require the microscope to reveal it; also each mosaic of the living animal or plant is a living mosaic or cell, in that it can absorb food, digest it, assimilate it, grow, and multiply in numbers. Cells, then, are the morphological or structural units which compose the bodies of all living creatures.

All animals and plants begin life as single cells, which, in the vast majority of cases, are microscopic in size. Those that remain single cells, and dissociated throughout life, are called Unicellular Animals (Protozoa) and Plants (Protophyta), and are to be seen mostly by the microscope alone; but those which, by multiplication and growth, form large numbers of cells that remain associated together as in the body of a bird or lizard, are called Multicellular Animals (Metazoa) or Plants (Metaphyta).

A cell ([Fig. 1]) is a nucleated lump of protoplasm, or cytoplasm, and most often of microscopic size and more or less covered on its exterior by, and holding in its interior, various products and formations resulting from its activity, which are called metaplasm. Since the protoplasm of a cell, under the microscope, presents a superficial resemblance to a minute speck of that jelly-like substance (albumen) which forms the white of an egg, it is often called an albumenoid substance. But it is very misleading to use such an expression, for protoplasm is not a single chemical substance of great complexity; but it is rather composed of a large number of different chemical substances of great complexity. Many of these substances, it is true, are albumenoid in character. The same is true as to the chemical complexity of the nucleus, which is a physically and chemically differentiated part of the protoplasm.

The protoplasm contains certain globulins, and also albumins and peptones; it also contains large quantities of nucleo-albumins, with other substances. The nucleus not only contains these same substances, but also nuclein and nucleo-proteids. It is important to state that nuclein consists of an albumin and nucleic acid.

Fig. 1.—Diagram of a Cell, highly magnified.

The protoplasm, structurally, is made up of threads forming a complex, sponge-like substance, or reticulum, called spongioplasm; and in the meshes of the spongioplasm is a more or less fluid-like substance known as hyaloplasm: suspended in the hyaloplasm are various kinds of living bodies known as plastids, besides various products resulting from the activity of the protoplasm and which are designated metaplasm.

In many cells the protoplasm has formed on its periphery a layer of metaplasm which is frequently called a cell-wall. This cell-wall prevents amœboid movements of the protoplasm, and a cell possessing it is said to be encysted.

In many cells, especially vegetable ones, will also be observed clear spaces termed vacuoles. These vacuoles contain water with various chemical substances held in solution, which serve the purpose chiefly of food-reservoirs.

The nucleus also is formed of threads called nuclear or chromatin threads (chromosomes), the interstices of which are filled with hyaloplasm or achromatin. In the nucleus can also be observed the nucleolus.

The protoplasmic and nuclear threads show various structural modifications in different regions and under different physiological states of the cell.

As will be observed later on, the nuclear threads are of special interest to the student of heredity. They may in one phase of cell-activity look like one thread forming an inextricable network, while in other phases they may look like thick, short, distinct rods.

The centrosome ([Fig. 1]), with its enveloping attraction-sphere, constitutes another fundamentally important part of the cell. It is especially concerned with the phenomena of cell division and multiplication.

Just as the living body consists of an infinitely complex figure of living mosaics termed cells, so the cell itself consists of an infinitely complex figure of still smaller living mosaics called, by Spencer, Physiological Units. These units have been given different names by various writers, viz.: by Darwin, gemmæ (gemmules); by de Vries, pangennæ; by Hertwig, idioblasts; by Weismann, biophors, etc., etc.

Like the atom of the chemist and the molecule of the physicist, the physiological unit of the biologist is merely at present an intellectual conception, yet it is, at the same time, an intellectual necessity and plays a very important part as the theoretical component of many vital questions. Just as the cells are the visible units of life, so the physiological units are the invisible units.

The physiological activities of cells are those that pertain to their nutrition and reproduction.

The nutrition of cells includes all processes that are subservient to their life and well-being, such as irritability, contractility, absorption of food, its digestion and assimilation, secretion, etc.

In consequence of the wonderful nutritive activities of cells, we may well speak of them as marvelous magicians. Hertwig, following Haeckel, speaks of many cells as being builders. In the same spirit, we can say that multitudes of cells are expert chemists, artists, sculptors, mathematicians, and so on, in that they make all the myriad chemical products of organic nature, such as spices, pigments, sugars, starches, acids, perfumes, and numerous other substances; they paint in colors that rival the hues of the rainbow; they construct all of the beautiful forms in the animal and plant worlds; and they draw lines as straight and curves as graceful as the most expert mathematician.

One of the most important reproductive activities of a cell is mitosis (see below). Mitosis essentially consists of a series of processes by which each nuclear thread of the nucleus splits longitudinally into two equal parts, and then these equivalent parts separate from each other, so that from the one nucleus we get two smaller nuclei. Then each of these smaller nuclei appropriates its share of the enveloping protoplasm, finally splitting it into two parts. Thus from the larger cell (nucleated piece of protoplasm) we get two smaller cells (two smaller nucleated pieces of protoplasm). In technical language, we say that the larger cell is the mother cell, and the two smaller cells that it has divided into are the daughter cells. In consequence of the method of mitosis, the two daughter cells very frequently are exactly like the mother cell, except in size. But by the absorption of nutriment, and through digestion and assimilation, they grow and finally become exactly like the mother cell. This is the simplest illustration of heredity. The reproductive process may be repeated very many times, so that from one cell we may get millions of cells.[1]

It is necessary to assume that the nutritive and reproductive activities of cells are based upon and controlled by the nutritive and reproductive activities of the physiological units, inasmuch as these are the ultimate living units.

In the activities of a cell the nucleus and protoplasm are intimately correlated with one another.

The nucleus is looked upon by the majority of cytologists as the formative center of the cell in a chemical, and also, consequently, in a morphological, sense. Active exchanges of material take place between the nucleus and the protoplasm during the nutritive processes of the cell. Possibly this may be altogether a chemical process, or possibly it may be due, as Hertwig suggests, to the migrations of the physiological units as carriers and elaborators.

In these exchanges, and in the upbuilding chemical activities (anabolism) of the cell, the nucleic acid plays a leading part. Here the nucleic acid in the physiological units of the nuclear threads, combines with albumins from the protoplasm, forming nuclein. Much of this nuclein, undergoing further elaboration, is passed into the protoplasm as one of its finished products (metaplasm). The more purely nutritive the activity of a cell, the more nuclein its nuclear threads contain; on the other hand, when the cell is in the phase of reproductive activity, the nucleus contains little nuclein, and is almost entirely composed of pure nucleic acid.

Fig. 2.—Stylonychia: c, an entire animal, showing planes of section; the middle piece of c contains two nuclei and can regenerate a perfect animal; a, and b, contain no nuclei,—they live and swim about for a while and then die.

That the nucleus is the formative center of the cell is indicated by the following, among many facts: If a unicellular animal, such as Stylonychia ([Fig. 2]), for instance, be broken up into several fragments, it will be observed that some of the fragments are nucleated and others non-nucleated. The nucleated fragments have the power of quickly healing the wounds on them, regenerating the missing parts, and thus restoring the mutilated fragments to perfect individuals. These nucleated fragments have the power to perform all the activities of the perfect animal. The non-nucleated portions, on the other hand, cannot undergo regeneration. They cannot digest food, or grow or secrete substances as the nucleated fragments can. They can simply live for awhile, responding to stimuli and moving about. They finally perish.

Having mentioned in a general way some of the wonderful powers of cells, it will be well now to describe briefly a few of the unicellular plants and animals that can be so easily obtained and studied in warm weather, and which may thus serve as illustrations of the powers of nucleated pieces of protoplasm or cells. Many unicellular plants and animals can be obtained in summer from the superficial ooze on the bottom of slow-running streams and also on the under surfaces of the leaves of water plants, a study of which will be of the greatest value and interest.

Amœba proteus ([Fig. 3]). This little unicellular animal, which belongs to the Rhizopod type, is very common in ponds and streams in warm weather. In the resting state it is spherical in form, but when active its form is as changeable as the fabled Proteus, hence its name, Amœba proteus. This little creature is a naked piece of protoplasm, with its outer layer differentiated into a firmer and pellucid part called the ectoplasm ([Fig. 3], ec); its interior, the endoplasm (en), is quite granular and much more fluid, the granular particles moving quite freely upon one another when the animal changes its shape. The superficial portion of the endoplasm is firmer than its more central parts, and graduates insensibly into the more consistent ectoplasm.

Fig. 3.—Amœba proteus: n, nucleus; cv, contractile vesicle; ec, ectoplasm; en, endoplasm; p, pseudopodia.

In the periphery of the granular endoplasm, and adherent to the inner surface of the ectoplasm, is a clearly defined nucleus (n). When most distinctly seen, it presents the appearance of a clear vesicle surrounding a solid and more or less spherical nucleolus. A contractile vacuole (cv) is also uniformly present, located in the endoplasm. The creature has the power of putting out projections (p) from the surface called false feet (pseudopodia). Sometimes the protrusion consists of ectoplasm alone, but more commonly endoplasm extends into it, when a current of granules will be observed moving from the more central portions of the Amœba into its protrusion, whilst from some other protrusion that is being withdrawn a similar current may set towards the center of the body, and thus the animal moves, in a creeping manner, from place to place. While moving about in this way the little animal comes across other one-celled creatures, such as Desmids and Diatoms, seizing them and forcing them through its ectoplasm into the endoplasm, where the nutritious parts are digested and assimilated. After the animal has taken its prey through its ectoplasm, no break in the continuity of the ectoplasm remains, but the parts immediately come together in a perfect manner. After it has abstracted all the nutriment from its prey, the Amœba casts away from it the parts that are indigestible.

Fig. 4.—Rotalia Freyeri: a many-shelled Foraminifer, or a colony of many single-shelled Foraminifera, with pseudopodia extended.

Foraminifera. These are little protoplasmic unicellular animals that have the power of secreting for themselves more or less complex envelopes composed of limestone. They may be single, as in Lagena, or composed of a number of individuals with the shells cemented together as in Globigerina or Rotalia ([Fig. 4]). They have played a part of vast importance in the geological development of the world. Their myriads of shells remaining at the bottom of seas millions of years after the little protoplasmic bodies have perished, they have been consolidated into vast expanses of limestone rocks, and finally uplifted into such formations as the huge chalk cliffs of England.

Osteoblasts and Osteoclasts. These cells are naked pieces of protoplasm, the latter much the larger and having many nuclei. They are concerned in some of the most interesting phenomena of many growing animals. Just as the Foraminifera have the power of forming complex aggregations of limestone shells, so the Osteoblasts have the power to construct the bones of animals. And when a bone is broken as the result of accident, these little cells do the mending. While the Osteoblasts are bone-formers, the Osteoclasts are bone-destroyers. It is very curious that little specks of living jelly, like these Osteoclasts, should have the power of destroying hard tissue like bone, but such is the fact. These Osteoclasts can, by their wonderful chemical processes, liquefy and absorb, and by these means destroy, ivory pegs that are driven into living bone. They are the agents by which the roots of children’s milk teeth are destroyed, so that the crowns of the teeth are shed and the way paved for the appearance of the permanent teeth. The wonderful activity of these little Osteoblasts and Osteoclasts is well exemplified in the growth and shedding of the antlers of deer. While these antlers are growing in the spring, they are covered with a delicate skin, technically called “velvet.” This velvet is very sensitive and quite warm from the nutrient blood circulating through it. In it are hundreds and thousands of busy, living Osteoblasts that work together under some mysterious, directing or coördinating agency, to build up the splendid beams, tynes and snags that constitute the antlers, which in many deer of the Rocky Mountains reach such a size that a man may walk under the archway made by setting the shed antlers up on their points. No hive of bees is busier or more replete with active life than the antler of a stag as it grows beneath the warm, soft velvet, through the agency of the Osteoblasts.

The building of the antlers by these little agents continues through the spring and summer. In the autumn the Osteoblasts cease their activity and die; the delicate, sensitive velvet dries and peels off, leaving the dead, hard, bony substance exposed, and they now become weapons adapted for fighting. This is the season when the stags challenge one another to single combat, the hinds standing timidly by to be taken by the victor as his mates. When the loves and battles of the autumn are over and the mating is completed, the antlers no longer serve a useful purpose, and they are shed. The shedding is accomplished through the agency of the bone-destroyers, the little jelly-like cells called Osteoclasts.

Bacteria are exceedingly minute specks of naked protoplasm. They are unicellular plants. Some of them are harmless to mankind; some are very useful to him, and others are his deadly enemies. Many of them are concerned in the production of the infectious diseases. They do so by elaborating various chemical products that are virulent poisons, hence these products are called toxines; when taken up by the blood, they are carried to various parts of the body. In this manner they cause the particular symptoms that are characteristic of a special infectious disease. Why is it that some persons, on exposure to an infectious disease, contract the malady while others similarly exposed do not? In other words, what gives immunity to disease? The explanation is probably as follows: Just as the invading bacteria have the power of secreting toxines, so the cells of the body, normally, have the power of elaborating chemical products that are antidotes to the toxines, and are appropriately called antitoxines. Infectious diseases and immunity from them, are the result of a contest between the invading bacteria and the protecting cells of the body. If the bacteria secrete toxines in greater quantities than can be neutralized by the cells of the body, we have disease; if the reverse occur, we have immunity.

The white blood-corpuscles (cells) also take part in this warfare. They have the power of traveling, in virtue of their amœboid movements, from the blood to the part invaded by the bacteria. Here a contest takes place between them, the corpuscle takes the bacteria into its interior, and either kills them or is itself killed. The result of this contest helps to produce either immunity or infection.

Tetanus bacillus is a cell shaped like a slender rod. It has the power of secreting a poison which, when introduced into the body, produces convulsions and other symptoms of lockjaw. These much resemble those induced by strychnine poisoning.

Bacillus diphtheriæ is an exceedingly small unicellular plant, and has the power of producing a poison called toxalbumin, which is analogous to the poison of certain venomous serpents. It is the speck of protoplasm through whose activity diphtheria is caused.

Many useful bacteria have the power of so acting on dead organic bodies as to decompose them, the three most conspicuous end-products of this decomposition being water, carbonic oxide and ammonia. When the dead bodies are decomposed in the soil there are other bacteria, in addition, that have the power of further acting on the ammonia, causing its oxidation and producing nitrous and nitric acids and their salts. The unicellular plants that bring about these changes are the nitrifying bacteria. Conspicuous illustrations of the functional activity of these little naked pieces of protoplasm are seen in the immense saltpeter beds of Peru and Chili, where, from the enormous fecal accumulations of sea-fowls, the immense quantities of nitrates are produced that supply the commercial world.

Fig. 5.—Difflugia Pyriformis.

Arcella, of which there are many species, is a unicellular animal whose protoplasmic body has secreted from its surface an enclosing “test” that is composed of a horny membrane, resembling very much in constitution the chitin which gives firmness to the integuments of insects. This creature is commonly discoidal in shape, with one face arched and the other flat, an aperture being situated in the center of the flat side through which the creature may thrust its pseudopodia or withdraw them. The surface of the testaceous covering is often marked with a regular but minute and attractive pattern. In Difflugia ([Fig. 5]), the test is somewhat pitcher-shaped, and is mostly made up (by the constructive activity of the protoplasm) of exceedingly small particles of shell and gravel cemented together. Many testaceous amœbans form tests of singular beauty and remarkable regularity. In some of the animals the minute plates of which the tests are formed have been picked up from the surface over which the animals crawl, and are cemented into various charming patterns; and in other cases they are formed by secretion from their own bodies. In Quadrula symmetrica the protoplasmic body has constructed a pear-shaped testaceous covering, of complete transparence-like glass, composed of a great number of square plates touching each other by their edges. The protoplasmic body of the animal does not entirely fill the test, the intervening space being occupied by a clear liquid and traversed by bands of protoplasm. A clear, large spherical nucleus is seen in the part farthest from the pseudopodia. It contains a dark and well-defined nucleolus. In front of the nucleus two contractile vesicles are to be observed. The pseudopodia in these creatures, it must be remembered, are not appendages, but lobate protrusions of the protoplasmic body, are few in number, rounded, short and broad.

Diatoms are unicellular plants, isolated or aggregated together, that have the power of constructing flint coverings, often of great complexity and charming pattern. The tracings on many of these flint coverings are so constant and small, that they are frequently employed for the purpose of testing the power of modern compound microscopes. In various parts of the world vast deposits of Diatoms have been discovered. The most remarkable of these for extent, as well as for the beauty and number of the species contained in it, is that on which the city of Richmond, in Virginia, is built, which is over thirty feet deep and extends for many miles.

Fig. 6.—Noctiluca miliaris. A, dorsal view; B, side view; n, nucleus; f, flagellum; a, entrance to atrium; b, atrium; o, œsophagus; r, superficial ridge.

Noctiluca miliaris ([Fig. 6]) is a very large unicellular, flagellate animal. It is spheroidal in form, and has an average diameter of not quite one-half a millimeter. It is just large enough to be observed by the unaided eye when the water in which the animal may be swimming is contained in a glass jar held up to the light. It has a tail-like appendage (flagellum) by which the animal moves about. Along one side of the cell is a meridional groove resembling that of a peach, and leading into a deep depression of the surface termed the atrium ([Fig. 6], B, b). It is from the shallow commencement of this depression that the flagellum ([Fig. 6], f) originates. At the base of the flagellum the depression sinks down to the mouth (o). A slightly elevated ridge (r) extends along the opposite meridian and commences with a bifurcation at that end of the atrium farthest from the flagellum. The mouth opens into a short œsophagus, which leads down directly to the central protoplasmic mass. The central protoplasmic mass sends off branching prolongations of its substance in all directions, the ramifications of which freely inosculate. The farther these ramifications extend out to the periphery, the thinner they become, until finally a protoplasmic network of extreme tenuity is formed immediately under the enveloping membrane of the cell. In addition to these ramifying prolongations, the central protoplasmic mass sends off a thin, broad, irregular extension to the superficial ridge and coalesces with it. Near the central protoplasmic mass is seen the nucleus (n).

The flagellum is a flattened, whip-like filament, having a striated appearance, and gradually tapers from the base to its extremity. It slowly bends over five or six times a minute to the mouth, and then, more slowly still, bends away again. It is through the movements of the flagellum that particles of food are driven into the mouth and down the œsophagus into the central protoplasmic mass. In this mass and its extensions the food is digested and assimilated.

This little one-celled animal has the power, through its special chemical activities, of manufacturing and emitting light. It is through the agency of myriads of these little creatures that the diffused luminosity of some seas is produced and can be observed at night. The Noctiluca is very transparent, and for this reason it is a particularly favorable subject for the study of its luminosity or phosphorescence. They can be obtained by the tow-net in unlimited quantities from the sea and transferred into a jar of sea water. Here they soon rise to the surface, forming a thick layer. If the jar be placed in the dark and agitated in the slightest degree, there is an instantaneous display of light, which is of a beautiful greenish tint. The light emitted by the Noctiluca is so vivid that it can even be observed in ordinary lamp-light. This phosphorescence is only of an instant’s duration, and a short rest is necessary for its renewal. The special locality for the formation of the phosphorescence is in the very fine protoplasmic network, which lines the external structureless membrane or cell wall. These wonderful little Noctilucæ may well be figuratively called the fire-flies of the ocean.

Fig. 7.—Gromia oviformis with protoplasmic threads (pseudopodia) extended and forming an elaborate network in which a captured unicellular organism is seen; d, diatom captured; p, protoplasm containing captured diatoms; s, shell.

Gromia oviformis ([Fig. 7]) is often found in fresh water adhering to confervæ and other plants of running streams. The protoplasmic body of this animal is enveloped in a chitinous covering that is egg-shaped and of a brownish-yellow color. It is about two millimeters in diameter. When the animal is quiet, no one would suspect its real nature, so much does it look like the seed of an aquatic plant. The testaceous envelope has a single round orifice at its more pointed end. The animal, when in an active state, pushes out the protoplasmic substance, which speedily gives off ramifying extensions, and these by further ramification and inosculation form a complicated network. The protoplasm of the animal also extends itself in such a way as to form a continuous layer on the external surface of the test. From this layer numerous protoplasmic threads may extend out, forming more or less complicated networks. By the alternate contraction and extension of its protoplasmic threads and networks, minute one-celled plants and animals are entrapped like flies in a spider’s web (d). When caught they are carried, by retraction of the protoplasmic thread-like pseudopodia, into the endoplasm in the test. Here the nutritious parts of the entrapped creatures are abstracted and assimilated. In transparent species, the indigestible parts, such as the silicious valves of diatoms, may be distinguished in the midst of the endoplasm, from which they are ultimately extruded.

When gromia oviformis reproduces by mitosis it gives off a bud (small cell), which finally separates from the parent form and constitutes a distinct individual. This process may be repeated many times, so that a great number of separate individuals may be formed, all of which lead detached and independent lives.

Most of the protozoa, which are produced by fission (cell division by mitosis), separate entirely from each other, as in Gromia; but in many of these unicellular animals, the new creatures produced by fission do not separate from one another, but remain more or less closely connected, and thus form colonies of Protozoans. These colonies are of the greatest interest, for they represent a lower stage of the cell colonies of the Metazoa (multicellular animals). They reproduce, in many cases, in a way which is strongly suggestive of reproduction in the Metazoa.

Microgromia socialis is a little unicellular animal, having a thin, nearly globular, calcareous shell that it secretes upon its surface. It multiplies by fission, and forms a number of distinct individuals which have the curious habit of fusing their pseudopodia and uniting into a more or less closely associated colony. The individuals sometimes remain at a distance from one another, but sometimes associate themselves together into a compact colony. These individuals are all alike, performing the same functions. There is no division of labor among the units, but they live practically an independent life. If the individual animals were detached from one another, they would live and build new colonies.

Codosiga umbellata is another unicellular animal—a flagellate Protozoan. It has a collar-like extension of its ectoplasm from the anterior extremity of its body, forming a sort of funnel from the bottom of which the thread-like structure (flagellum) arises. The vibrations of this flagellum cause a current of the surrounding fluid to set into the funnel so that particles of food reach the soft protoplasmic substance which serves as a mouth. The nucleus is seen near the base of the collar. Near the posterior extremity of the body two contractile vesicles are to be observed. This posterior extremity of the animal has a cylindrical extension of its ectoplasm by which it attaches itself to an object. This protozoan multiplies by longitudinal fission. In some species the animals separate completely from one another and lead entirely independent lives. But in Codosiga umbellata the fission does not extend through the cylindrical extension, so that a group of animals are associated in a colony.

Rotalia or Globigerina. The shelled amœba (Lagena) gives off a bud (unseparated cell) which grows to the full size, secretes its calcareous shell and remains connected with the parent form. This process may be repeated a number of times until a colony of shelled amœbæ, of varying pattern in different species, may be formed and permanently associated together ([Fig. 4]). The individual amœbæ are all alike and perform the same functions. There is no division of labor, no specialization, among them. If the individual animals could be separated from one another they would live and build new colonies.

Pandorina morum forms a small colony of sixteen cells (solid sphere) of mulberry-like shape and enclosed in a common gelatinous envelope. Each cell in the mulberry mass bears two flagella on its peripheral end. These project out beyond the surface of the gelatinous envelope, and are agents for locomotion of the colony. The cells in the colony are all alike. There is no division of labor among them. They all act alike. The cells (flagellate protozoans) of the colony may reproduce in two ways. Each animal in the colony may subdivide into sixteen smaller units, each of which by growth and multiplication may form a new mulberry mass, a new colony, each unit of which acquires two flagella. Or two of the small units may amalgamate (conjugate), and then develop (by fission) into a new colony. The conjugating units are nearly of the same size and look very much alike.

Volvox globator is a spheroidal shaped colony (hollow sphere) of unicellular flagellate animals, about one-half a millimeter in size. It was formerly supposed to be a fresh-water Alga. It is now known to be a colony of Protozoans. All the animals in this colony are not alike. There is a division of labor among the cells, for some are merely vegetative, serving purposes of nutrition, and having no reproductive powers; while other members of the colony are purely reproductive animals. Furthermore, there is quite a marked specialization of the reproductive cells. Those reproductive cells that may be spoken of as the female cells are large and non-motile encysted cells. The male cells are small and actively motile, in that they have two flagella developed on them. A small flagellate male cell penetrates the large encysted female cell, and as the result of this conjugation, fission takes place repeatedly, and a new colony of flagellate protozoans (volvox) is formed. Volvox approaches very suggestively towards the type of animals known as Metazoan.

In order to comprehend, in some measure, the transition from Colonial Protozoa to Metazoa, it will be well for the reader to study a typical sponge. For a long time the Porifera (Sponges) were looked upon as compound Protozoa (colonial Protozoa), but while they are nearer the Protozoa than any of the other types of Metazoa, their position in the animal series is unquestionably among the Metazoa. The Sponge, like the rest of the Metazoa, develops from a fertilized egg by a process of cell multiplication, differentiation, gastrulation, etc.

CELL REPRODUCTION BY MITOSIS.

The multiplication of cells plays a part of such fundamental importance in Evolution, and therefore in Embryology and studies in Heredity, that it is necessary to study the subject somewhat in detail. It is a wonderful process, and is worthy of very careful attention.

Fig. 8.—Diagram illustrating Mitosis. A, the cell commencing activity; B, C, D, phases in the formation of the spindle and the chromatin loops or V’s, also showing that the mother V’s have split into daughter V’s; D, the chromatin loops forming the equatorial plate, chr; E, F, G, separation of the daughter loops (daughter chromosomes) and their passage towards the poles of the spindle, thus forming daughter nuclei; H, I, division of the protoplasm so as to form two daughter cells; at, attraction sphere enclosing a centrosome; n m, nuclear membrane; chr., chromatin threads; p, protoplasm; c w, cell wall; sp, spindle.

The process by which one cell (a mother cell) divides into two cells (daughter cells) is called mitosis, and is inaugurated by the centrosome ([Fig. 8], A, at). The centrosome divides into two centrosomes, which at first remain close together ([Fig. 8], B, sp), and then gradually separate from one another. Each centrosome becomes the center of a system of fine achromatin fibers arranged round it in a radiating manner and forming what is called the attraction sphere; also, at the same time, a spindle-shaped bundle of achromatin fibers, called the spindle ([Fig. 8], B, sp), extends between the centrosomes. In the meantime, important changes have been taking place in the chromosomes (hereditary threads) of the nucleus. The chromosomes, which at first are arranged in an apparently inextricable tangle or network, frequently assume U-shaped or V-shaped forms ([Fig. 8], C, chr), and the nuclear membrane disappears. Sooner or later each chromosome splits longitudinally into two daughter chromosomes, with which the achromatin fibers of the spindle become connected ([Fig. 8], D). In this phase of mitosis the split V-shaped chromosomes form a single group called the equatorial plate (chr), and extend across the axis of the spindle. It is to be observed from the diagrams in the figure, that one of the centrosomes has traveled to the opposite pole of the nucleus, thus causing the achromatin fibers of the spindle to extend across the original site of the nucleus. The equatorial plate of split V-shaped mother chromosomes (hereditary threads) thus divides the fibers of the spindle into two parts, one half extending from one centrosome to one group of daughter chromosomes, while the remaining half extend from the other centrosome to the other group of daughter chromosomes. Soon the achromatin fibers of the spindle contract, and in this way separate the two groups of daughter chromosomes, so that one group is drawn towards one centrosome, and the other group to the other centrosome ([Fig. 8], E, F, G, H). After the two groups of daughter chromosomes have been drawn to their respective centrosomes, each group assumes the tangle or network phase like the nucleus of the mother cell, and an investing nuclear membrane reappears for each ([Fig. 8], I). Thus from the mother nucleus of the mother cell we get two daughter nuclei (I). In a further phase of the mitotic process, a furrow appears on the surface of the protoplasm and surrounds it in the form of a ring. This furrow is in a plane at right angles to the long axis of the spindle, and gradually deepens until the protoplasm is divided into two parts, each segment of protoplasm containing its own nucleus and centrosome; in short, the mother cell has divided into two daughter cells (I).

It will thus be observed that the centrosomes and their achromatin fibers are a beautiful mechanism by which the heredity threads (chromosomes) are exactly divided into two equivalent halves.

There are some cells (Amœba proteus, for instance,) which divide in a much simpler manner than by mitosis; in these there is no complicated rearrangement of the chromosomes and no disappearance of the nuclear membrane, the nucleus simply becoming separated into two parts (Amitosis).

The Human Ovum. The human ovum is a typical cell about one-fifth of a millimeter in size and spherical in shape. It is a nucleated piece of protoplasm possessing an enveloping cell-wall (metaplasm). The protoplasm contains nutrient material or yolk (metaplasm). The maturation of the ovum essentially consists in throwing out half of its chromosomes. In doing this the nucleus (mother) approaches the surface of the protoplasm ([Fig. 9], A), and divides, by mitosis, into two daughter nuclei; then the unripe ovum divides into two cells, but of very unequal size. This process is repeated a second time. Thus two small cells are formed which are known as the polar bodies ([Fig. 9], A, B, pol. b). The large cell remaining after the formation of the two polar bodies is the mature ovum (B). Its nucleus, which recedes towards the center of the protoplasm, is called the female pronucleus ([Fig. 9], B, f. pr.). The pronuclei of mature ova differ from the nuclei of all the other cells of the body in that they only contain half as many chromosomes (hereditary threads).

The ripe spermatozoid (a flagellate sexual cell of the male) corresponds to the ovum of the female. It also contains a pronucleus having only half the number of chromosomes that the other cells of the adult body possess.

Fertilization. The male sexual cell (spermatozoid) is vastly smaller than the female sexual cell (ovum). Having a flagellum ([Fig. 9], B, s), it moves about, like a tadpole in water, and seeks the ovum. When it comes in contact with the ovum it penetrates into its interior (usually only one doing so), as indicated at ([Fig. 9], B, s). The tail or flagellum of the spermatozoid fuses with the protoplasm of the ovum, and disappears from view. Its pronucleus (C, m. pr.), accompanied by its centrosome (C, m. c.), approaches the female pronucleus (f. pr.) of the ovum ([Fig. 9], C, D). Finally the male and female pronuclei coalesce to form a single nucleus ([Fig. 9], E, n. os.). The centrosome of the ovum persists for awhile and then disappears; that of the spermatozoid remains in the ovum, and is the agency by which cell multiplication, through mitosis, takes place.

Fig. 9.—Diagram illustrating the maturation and fertilization of the human ovum. A, one polar body is formed and a second is in process of formation; B, both polar bodies are formed and a spermatozoid is penetrating the ovum; C and D represent the approach of the male pronucleus towards the female pronucleus; E indicates the amalgamation of the two pronuclei to form the nucleus of the oösperm (segmentation nucleus); pol. b, polar bodies; pol. c, centrosome of the polar body; chr. p, chromatin of the polar body; f pr, female pronucleus; p, protoplasm; p p, peripheral protoplasm (but not cell wall); f c, female centrosome; m c, male centrosome; m pr, male pronucleus; n, os, nucleus of the oösperm (first stage of a human being).

The cell resulting from the coalescence of the male pronucleus with the pronucleus of the ovum, and which is only one fifth of a millimeter in size, is the first stage in the existence of a human being. Man thus starts his career as a Protozoan-like creature,—as a unicellular animal. The fertilized ovum is called the sperm-egg (oösperm), and contains now the normal number of hereditary threads (chromosomes); for those of the male have been added to those of the female.

Segmentation of the Oösperm. The fertilization of the ovum imparts to it a wonderful stimulus, so that the oösperm divides, by mitosis, into two cells, these two into four, the four into eight, the eight into sixteen, these into thirty-two, and so on repeatedly, until a large number of comparatively small cells are formed ([Fig. 10]). This mass of cells is spherical in shape, and the little round cells towards the surface project in such a way as to give to the mass an appearance somewhat similar to the fruit of the mulberry, whence it is termed the mulberry body or morula ([Fig. 10], 4). In the morula stage of his existence man resembles the solid colony of protozoans represented by Pandorina. The cells of the morula next become arranged regularly in a single layer at the circumference, by which the embryo assumes the form of a hollow sphere, and is known as the blastula ([Fig. 10], 5). This phase of man’s existence is quite suggestive of Volvox.

Soon one side of the blastula is invaginated or pushed in, as one would push in one side of a hollow india-rubber ball. The result of this invagination (called gastrulation technically) is the formation of a sort of cup. This is the gastrula ([Fig. 10], 6) phase of man’s existence. It is a higher and fundamentally different phase of existence than either the morula or the blastula. It corresponds, not to a Protozoan, but to the higher Metazoan. It possesses the fundamental anatomical qualities of a low Cœlenterate (Polyp).

The mode of gastrulation is different in man from that just described, and varies in different animals, but the essential point common to all is the formation of a double cellular-membrane; the outer membrane being called epiblast or ectoderm, and the inner one hypoblast or endoderm, the enclosed cavity being the primitive digestive cavity. These two layers are the primary germinal layers. A third layer is subsequently formed between them, by the agency of one or both of them, and is called the mesoblast. From these three simple membranes, which are composed exclusively of cells, are formed all the complex tissues and organs of the adult man. The epiblast develops into the nervous system (brain, spinal cord, and nerves), and the cuticle, hair, etc. The hypoblast develops into the cellular parts of the digestive canal, the liver, lungs, etc. The mesoblast develops into the muscles, bones, ligaments, blood-vessels, etc. To trace the details of this evolution of a human being from the microscopic oösperm is a fascinating and instructive study, but is beyond the limits and purpose of this little book. It will well repay further careful study by the reader.

A careful study of the life-histories of the few unicellular animals and plants mentioned in the preceding pages will help to let one realize the wonderful powers of nucleated pieces of protoplasm (cells). If isolated pieces of protoplasm can accomplish so much, one will not be astonished to learn that many diverse cells associated together in intimate correlations, as occurs in the higher animals, may accomplish results that are profoundly interesting and marvelous. As far as anatomy and physiology alone can reveal, it is the result of millions of cells acting together that makes possible the existence of such a living, sentient, thinking creature as man or the highly intelligent elephant or any other multicellular animal. It is due to the mysterious powers of protoplasm that one little microscopic cell, like the fertilized ovum of a woman, is able to hold all the heritages of the race, and gradually unfold them as it builds up the body into myriads of diverse cells intimately associated together.

All of the wonderful results of Embryology are accomplished through cell-multiplications, cell-differentiations, cell-associations, invagination and evaginations of cell-groups (tissues or organs), and unequal growth of parts (cells or groups of cells).

Fig. 10.—Segmentation of the fertilized ovum and Gastrulation: 4, morula; 5, section through blastula showing hollow sphere; 6, gastrula showing outer layer of cells (epiblast) and inner layer (hypoblast); the 6 is at the mouth of the cavity (enteron) of the gastrula.

In concluding this brief but, we hope, useful study of a few selected cells, we may say that an eminent English physiologist has made the statement that a student who has not looked through the microscope and observed the circulation of the blood in the web of a frog’s foot is not fit to study medicine. However beautiful, fascinating and instructive the sight of this circulation may be, we are tempted to make the assertion that the student who has not looked through the microscope at some of the superficial ooze from the bottom of any slow-running stream, in summer, and observed the structure and actions of that wonderful little unicellular animal, the Amœba proteus, is still less prepared to study medicine. In this little speck of protoplasm the problems of life are reduced to their simplest forms, for all higher plants and animals may be regarded as groups of more or less modified amœbæ peculiarly associated together. In our studies of the amœba we will be forcibly reminded of a very clever trick which is practiced in India and is called the mango-trick. In this trick a seed is put into the ground and covered up, and after divers incantations a full-blown mango-bush appears within five minutes. We have never met any one who knew how this thing was done, nor have we ever seen a person who believed it to be anything else than a conjuring trick. So it is with the amœba, a beautiful and fascinating trickster of nature. We understand some of its activities, interesting and exceedingly instructive, but there are many others beyond our ken. We see the commencement and ending of many of its chemical activities, but there are numerous other intermediary processes that take place in the hidden recesses of the protoplasm, and concerning which we know nothing. It may fall to the lot of some reader of this little book, as a patient and keen observer, to unravel some of these mango-like tricks of the amœba or other unicellular creature.

SECTION II.
HEREDITY WITH VARIATION.

HEREDITY WITH VARIATION.

That an offspring always inherits from its parents many of their characteristics is well known; that it always varies, more or less, from them is also equally well known. Heredity and variation are twin forces that play upon every creature, holding it rigidly true to the parental type or compelling more or less divergence therefrom, according to the strength of the one or other power; so that every creature is the resultant of the activities of these two great parallel forces. Variation is coextensive with heredity, and every living creature gives evidence of the existence of variations.

Examples of Variations. No two leaves on a plant are exactly alike; no two children of the same parents give a perfect resemblance; no two individuals of the same species are molded in precisely the same pattern; of the thousands and thousands of faces that we observe in a city in the course of a year, each has some distinctive peculiarity.

The trained eye of the gardener recognizes each hyacinth among hundreds of bulbs; of the shepherd, each sheep in his flock; of the Laplander, each reindeer crowded in his herd like ants on the anthill. In a flock of 1,000 sheep each mother can even recognize a variation in the voice of her own lamb, all alike to us.

Every part of an animal is subject to variations, not only in bodily structure, but also in habits and instincts, and these variations are large in amount, numerous and diverse in character. Many observations, experiments and measurements that have been made at various times attest the truth of this assertion. Not only do variations take place in animals and plants under domestication, but also in the wild state.

Illustrations of Heredity. Mental heredity can be illustrated by studying the genealogies of such persons as Aristotle, Goethe, Darwin, Coleridge, Milton, etc. Probably the Bach family, of Germany, supply one of the best illustrations of the inheritance of intellectual character that we know of. The record of this family begins in 1550, lasting through eight generations to 1800. For about two centuries it gave to the world musicians and singers of high rank. The founder was Weit Bach, a baker of Presburg, who sought recreation from his routine work in song and music. For nearly two hundred years his descendants, who were very numerous in Franconia, Thuringia, and Saxony, retained a musical talent, being all church singers and organists.

When the members of the family had become very numerous and widely separated from one another, they decided to meet at a stated place once a year. Often more than a hundred persons—men, women and children—bearing the name of Bach were thus brought together. This family reunion continued until nearly the middle of the eighteenth century. In this family of musicians twenty-nine became eminent.

Inheritance of moral character is well established. Heredity, in its relation to crime and pauperism, has been thoroughly investigated by Mr. Dugdale in his most instructive little work entitled The Jukes. In this work the descendants of one vicious and neglected girl are traced through a large number of generations. It reveals that a large proportion of the descendants of this woman became licentious, for, in the course of six generations, fifty-two per cent. of the females became harlots and twenty-three per cent. of the children were illegitimate. It shows also that there were seven times more paupers among the women than among the average women of the State, and nine times more paupers among the male descendants than among the average men of the State.

The inheritance of physical peculiarities is so obvious as to need no illustration. Among the ancients the Romans stereotyped its truth by the use of such expressions as the labiones, or thick-lipped; the nasones, or big-nosed; the capitones, or big-headed; and the buccones, or swollen-cheeked, etc. In more recent times we read of the Austrian lip and the Bourbon nose.

Questions of heredity and variation are cytological ones—that is, questions of the anatomy, physiology, physiological chemistry, and pathology of cells. The most important part of a cell, as far as these questions are concerned, is the nucleus. The nucleus is the physical basis of all the heritages of an organism, from the simplest to the most complex. The nuclear threads may, therefore, very appropriately be termed the hereditary threads, or, collectively, the hereditary mass; and the physiological units in them the hereditary units. The nucleus is of fundamental importance in the reproduction or multiplication of both unicellular and multicellular animals and plants.

In unicellular creatures multiplication may take place by fission and by conjugation. Both of these processes can be studied by observation of the infusorians. Maupas’s beautiful investigations on these unicellular animals have demonstrated that multiplication by fission may proceed to a prodigious extent for many generations, but that a time comes when the process fails, and the species will become exhausted and die out unless there is a rejuvenation of it by conjugation of individuals. In conjugation two individual infusoria come in apposition with each other, the nucleus in each undergoes subdivision. They reciprocally exchange part of their nuclear contents so that each infusorian comes to contain hereditary threads of two distinct individuals. From these rejuvenated (or fertilized) individuals multitudes of others may be derived by fission until exhaustion again takes place.

Multiplication in multicellular creatures may be accomplished by budding (which is allied to fission), and is exemplified in the plant, hydra, the queen bee (parthenogenesis), etc., and by fertilization (which is allied to conjugation). A knowledge of the phenomena of fertilization of the ovum by the spermatozoid is essential to any understanding of the problems of heredity and variation in mankind. The nuclear threads of the ovum are its hereditary threads—the groups of maternal hereditary units; likewise, the nucleus of the spermatozoid contains the paternal groups of hereditary units.

Fertilization. In fertilization, the spermatozoid (a nucleated flagellate cell) penetrates the ovum (a nucleated, encysted cell), its protoplasm mixes with that of the ovum, and its nuclear threads come into relation with the nuclear threads of the ovum; so that the fertilized ovum (a new creature, a veritable microcosm) is still a nucleated cell, but one in which the nucleus is compound, is hermaphroditic, in that it contains maternal and paternal threads—that is, maternal and paternal hereditary units which constitute its hereditary mass.

It will be convenient to speak of the maternal and paternal hereditary units in the fertilized ovum as ancestral hereditary units.

This hermaphroditic cell passes through complex phases, illustrated by embryology, to the adult. In doing so this hermaphroditic cell (mother cell) first divides into two smaller cells (daughter cells). The mother cell divides in such a way (by mitosis) that one-half of its nucleus and part of its protoplasm goes to one daughter cell and the other half of the nucleus, with the remainder of the protoplasm, goes to the other daughter cell. It is an interesting fact that although the amount of protoplasm which goes from the mother cell to the two daughter cells may be unequal at times, yet the amount of the nucleus in one daughter cell is always exactly equal[2] to that in the other; so that each daughter cell contains maternal and paternal hereditary masses of equal quantity and quality, being in fact one-half that of the fertilized ovum. In consequence of the fact that each hereditary unit in the nucleus of the daughter cell can absorb nutriment and grow, it comes about that the nucleus of each daughter cell attains to the size of that of the mother cell. The enveloping protoplasm of the nuclei also grows to a greater or less extent, so that the cells as a whole grow. These two daughter cells go through the same process and form other daughter cells, and so on through all the mitoses of development, until all the myriads of cells of a living organism are produced, each of which contains maternal and paternal hereditary masses of equal quality and quantity, and also of the same character as that of the fertilized ovum whence they are all derived.

Apart from their activity in absorbing nutriment and growing, the great majority of the hereditary units in the nuclei of the forming cells remain latent. But some of the hereditary units in each cell produced are active. They multiply, grow and migrate out of the nucleus, and get among the units of the enveloping protoplasm. During this activity they undergo physical and chemical changes and effect corresponding differentiations in the protoplasm of the cell. Thus, through many mitoses and many differentiations of the protoplasm of cells, we finally derive from the fertilized ovum all the cells that constitute the adult body, such as muscle cells, glandular cells (as liver cells, kidney cells, etc.), nerve cells, skeletal cells (as bone, cartilage, and connective-tissue cells), and the ova and spermatozoids. According to this theory, the nucleus of each cell in the adult animal or plant is pure hereditary mass, exactly like that in the nucleus of the fertilized ovum; but the protoplasm of each adult cell that envelopes the nucleus may differ greatly in different cell groups, as in muscle, nerve, cartilage, and the like. Of course, the protoplasm of those cells that develop into the ova and the spermatozoids has differentiated along such lines as to become like that of the ovum and spermatozoid, the junction of which formed the fertilized ovum. These statements hold true for plants and the lower animal organisms, although they cannot be verified for the higher animals. More than likely the pure hereditary masses are present in the body (somatic) cells of the higher organism in latent conditions, but are unable ever to be developed owing to the greater specialization of these cells. It is thus seen that all the cells of many animals and plants can perform their own special functions and at the same time contain all of the hereditary units of the complex organism in a state ready to develop under favoring conditions.

Since the ova and spermatozoids are cells specially differentiated for the purpose of propagating the species by sexual generation, and since their conjugation produces the germ of a new creature, they may very appropriately be spoken of as germ cells. Since all the other cells of the adult form the great bulk of the body that envelopes and protects the germ cells, they may be termed the body cells, or somatic cells.

Suppose that ova, containing maternal and paternal groups of hereditary units, are fertilized by similarly complex spermatozoids, and the process is repeated generation after generation. There will come a time when the fertilized ovum will have a highly complex nucleus composed of many different ancestral groups of hereditary units.

One often hears the expression that a child is a chip of the old block; but this is only a very partial truth, for a child is preëminently a composite chip of many old blocks.

The complex nucleus of the fertilized ovum may be compared to a modern Italian building which has been constructed of material—a column here, a cornice there, a lintel yonder—gathered from different classic buildings of varying antiquity. In view of the increasing number of ancestral groups of hereditary units that must have accumulated in the nuclei of ova in the course of time, there must necessarily, for mechanical reasons, have arrived a period when these nuclei could receive no more of them by fertilization, unless natural selection should develop some saving device; hence we have, possibly, an explanation of the phenomena of maturation in ova (the reducing process of Weismann and Hertwig). Here the ovum, prior to fertilization, undergoes mitosis twice in succession, by which the polar bodies are formed and the hereditary mass is diminished by one-half (the mature germ cells having only one-half the number of nuclear threads that the body cells possess). A homologous process takes place in the maturation of spermatozoids. Fertilization increases the amount of the hereditary mass in the ovum to the original quantity, and thus restores the number of nuclear threads to the specific number. All the body cells derived from the fertilized ovum possess, also, the specific number of threads.

This union of two distinct hereditary masses is called amphimixis as well as fertilization.

Maturation and amphimixis or fertilization are the source of many variations in the body, good and bad, beautiful and ugly, geniuses and monstrosities; because, in the commingling of distinct hereditary masses, there is a struggle for existence between the hereditary units and a survival of the fittest.

In this struggle some of the hereditary units are strengthened so that heritages may be augmented; some mix so that there may be a blending of characteristics; some are mutually exclusive; some are prepotent; some are neutralized; some are destroyed; some lie dormant (latent) for varying lengths of time, and some are so altered as to produce much modified forms; and thus the possibilities of combination, reactions and modifications of the hereditary units, and therefore heritages, are almost endless.

The augmentation of heritages in the fertilized ovum is well displayed, for instance, where fleet horses are bred with fleet ones, until, by careful selection, generation after generation, a progeny may be secured much more swift than the original stock from whence they were derived. In the same way good milch cows have been produced.

The mixing of hereditary units and blending of heritages is shown in the color of the skin, as where a mulatto child is born to a negress by a white father; mutually exclusive heritages are well illustrated in the color of the eyes, as where a child has either the blue eyes of one parent or the black of the other, but never any blending of the colors; this may also be illustrated where the white game bird and black one are crossed, the young being either white or black, but never blended. Prepotency is illustrated where the silky variety of fan-tailed pigeon is mated with any other small-sized variety of pigeon, for the silkiness is invariably transmitted. A most interesting case of prepotency in mankind, mentioned by Ribot, is that of Lislet-Geoffrey, an engineer in Mauritius. He was the son of a very stupid negress and an educated white man. In physical constitution he was as much a negro as his mother; he had the woolly hair, the features, the complexion, and the peculiar odor of his race. He was so thoroughly a white man as regards intellectual development that he succeeded in vanquishing the prejudices of race, so strong in the French colonies, and in being admitted into the most aristocratic houses. At the time of his death he was Corresponding Member of the Academy of Sciences.

In this, it will be observed, we have prepotency in the mother’s physical constitution, and in the father’s intellectual characteristics.

The struggle of heritages in the impregnated ovum may lead to such structural changes of the nucleus, and therefore of the cell, as to develop the most marked variations—such variations as the biologists call sports.

In the latter part of the eighteenth century the farmers of Massachusetts had flocks of ordinary sheep on their farms. These sheep were continually jumping fences and getting on neighboring farms. They were the source of many disputes and much irritation between neighboring farmers. Finally, one of the sheep had a lamb which, when grown, displayed well-marked peculiarities (a sport). It had a longer body than the ordinary sheep and shorter legs, which were bowed. It was noticed that this sheep could not get over the fences. The cute Yankee farmer, noticing this valuable peculiarity, carefully preserved this peculiar sheep, and from it was ultimately derived, by careful selective breeding, a special variety known as the Ancon sheep.

The germinal variations resulting from the mixing of two separate hereditary masses by impregnation find their expression in the most varied qualities of the minds and bodies of developing children. If the variations are not especially marked, they are looked upon as normal and attract no special attention.

But if the variations are so pronounced as to compel attention, and at the same time it is known that they are useful, they are spoken of as talents, or, on the other hand, if they are harmful or useless, they are designated as pathological or monstrosities.

These are truly what the biologists call sports; and to those classes of sports that occur as specially gifted in human culture, in the varied fields of science, art, or literature, we assign such a person as Shakespeare, and call the remarkable variations embodied in him genius. On the other hand, such variations as lead to certain forms of pigmentary degeneration of the retina, and to Daltonism, to dyschromatopsia and achromatopsia, to certain supernumerary glands, polydactylism, and such like, which are either useless or harmful, we designate as pathological cases or monstrosities.

Hereditary units (the carriers of heritages) may be latent—that is, they may appear late in life, or in the offspring, or, still again, in remote descendants; in the latter cases the heritages are spoken of as reversional or atavistic. Latent hereditary units may very usefully be compared to dormant seeds buried in the ground. It is stated that buried seeds may lie dormant for many years, so that when a plot of ground is plowed deeply and upturned, plants that have not been seen there within the memory of one will often make their appearance and flourish. The hereditary units are veritable living seeds, that, under certain and often unknown stimuli, grow and unfold their heritages as do the buried seeds.

Latent heritages are well illustrated by a study of secondary sexual characters as developed at puberty.

Among our barnyard fowls, the hens often, when they have atrophy or degeneration of the ovaries, although up to this time they have laid eggs for years, stop this function, put aside the plumage and appearance proper to their sex, and don more or less completely the garments of the rooster. Thus females have latent in them many secondary sexual characters of the male. For similar reasons the male develops, occasionally, female characters.

This latency is illustrated again in deer. In most species of the deer tribe the males alone possess antlers, yet it is a well-known circumstance that in females with degenerations of the ovaries rudimentary horns that are never shed appear. A study of congenital color-blindness illustrates beautifully latent heritages, showing how the females of one generation may be free from the malady and the males of the next afflicted.

A study of the regeneration of lost parts in various animals and plants illustrates well the latency of many hereditary units. A cutting made from the willow and planted sends out roots and finally reaches the dimensions of the adult tree. Here the body cells of the stem evidently contained, in a latent condition, many hereditary units in their nuclei, which became active through the special stimulus of being planted as a cutting. Adult plants can be raised from the cuttings of many other plants.

If the garden worm is cut in two, the head-part will reproduce the tail-portion. If the little fresh-water polyp, Hydra, be cut into a number of pieces, each segment will reproduce a perfect animal. Many lizards, after losing their tails by violence, manufacture new tails through the agency of the latent hereditary units contained in the body cells of the stump left. If the tentacle or “horn” of a snail, which contains an eye with a perfect lens and retina, be cut off, the animal can reproduce another one with a perfect eye, and this can be repeated a number of times. Often newts, when fighting with one another, or lobsters when fighting, lose a leg or a claw. These highly organized animals have the power of creating new limbs, making bones, ligaments, muscles, nerves, cuticle, and so on. All of this is done through the hereditary masses in the nuclei of the body cells at the site of the injury.

A study of the phenomena of polymorphism in hydroids and insects will beautifully and most interestingly illustrate latent hereditary units.

Much that is speculative and fanciful is included under the subject of atavism, and the safest plan for pathologists and biologists, in considering any abnormality, is to remember a golden rule of Gegenbaur, that only those structures are reversional which are taxonomically not far distant or phylogenetically not very old. Embryology is also a very important check in considering such subjects.

In mankind supernumerary limbs and digits, microcephalia and micrencephalia, have been looked upon as reversions to the simian type.

Lombroso, in contrasting the criminal with normal man, looks upon his homo delinquens as an illustration of atavism, contrasting with homo sapiens. But, as Ziegler, in his Pathology, well observes, many writers have gone too far in this respect, and have characterized as atavistic formations various acquired pathological formations and fresh variations of germ cells.

I think one can safely say that supernumerary ribs and those supernumerary nipples and mammary glands along the line of the deep epigastric and internal mammary arteries are truly atavistic structures; also certain muscles normally belonging to those mammalia which come near to man in the scale of relationship, and which appear in man as muscular variations, are reversional.

Children are often born with pigmented hairy patches on their bodies known as moles; sometimes these hairy moles are only of the size of a split pea, in other cases they are several square inches in area, while in rare cases almost all of the trunk may be thus covered. Although many similar pathological cases are often but marked variations called sports, yet the illustrations mentioned are undoubtedly reversional. Of the multitudinous illustrations of atavism that could be mentioned I wish to refer to but one more case.

The conjunctiva is a modification of skin, and frequently proclaims its ancestry by reverting to its original form. It is by no means a very rare event to see a patient having a patch of hair-covered skin growing upon the ocular conjunctiva. While a clinical assistant at the Royal Ophthalmic Hospital in London, we saw one such case, and Dr. Treacher Collins, the pathologist of that eye hospital, has stated that about twelve cases are seen there annually of this pathological condition, which is atavistic, according to Sutton, although it seemingly violates Gegenbaur’s rule about phylogenetic remoteness, and may be looked upon by some as a pathological illustration of a sport.

In speaking of inheritance, we should carefully discriminate between heredity and pseudo-heredity. Physicians constantly write of tuberculosis, lepra, smallpox, and syphilis as hereditary; but it is incorrect and misleading to do so. When a person has syphilis, say, from the earliest existence—that is, from the fertilized ovum by transmission of a syphilitic microbe through the germ cells of the parents—this should be designated by its proper name as congenital bacterial infection. This is totally different from the hereditary qualities that flow from the structural equilibrium following the commingling and struggle for existence of multitudes of hereditary units.

The one set of hereditary qualities is purely germinal, while the other is germinal profoundly modified by the presence of an infecting microbe. Of course, to the extent that any toxines that are secreted by the bacteria may cause permanent structural changes in the germ-cells, to that extent may the germinal characteristics be transmitted and become hereditary.

Many instances of infection of the child in utero have been reported in cases of endocarditis, scarlet fever, and smallpox; and there can no longer be any doubt, from experimental investigation and recent observation, that pneumococci, typhoid bacilli, anthrax bacilli, and pus cocci are able to pass to the fœtus through the placenta. But the diseases that develop in this way can be called hereditary with even less semblance of correctness than in the case of the fertilized ovum that is invaded with a microbe.

All of these cases are illustrations of pseudo-hereditary transmission, and should, for the sake of clearness and accuracy, be spoken of as prenatal infections.

So far as the problems of heredity and variation are concerned, we may say that the life cycle begins and ends with the germ cell. Insects lay their eggs in old age; among plants the annuals flower but to die; in higher creatures the cessation of the procreative power often marks the beginning of bodily decline.

Bearing in mind that the human body consists of two great classes of cells, germ cells and somatic cells, the following scheme will be found very useful in discussing heredity with variation—viz.:

Just in proportion as fertilized germ cells during the mitoses of ontogeny give origin, among the somatic cells, to other germ cells that are structurally, and therefore physiologically, like themselves, just to that extent do we have heredity; on the other hand, just to the degree that the new germ cells which are produced are unstable, to that degree also do we meet with variations.

ENVIRONMENT.

In zoölogy the environment of an organism means the sum-total of the conditions of life that surround and affect it, such as food, air, water, climate, etc.

We have already stated that as far as evolution is concerned the structures of fundamental importance in an organism are the Germ-Cells; therefore, for our purposes, we will define environment to be the sum-total of the conditions that directly or indirectly influence in any way the germ-cells, by which variations in them may be produced, or through which stability may be maintained.

There are two great classes of environmental factors that bring about variations in the germ-cells. One of these classes acts directly on the germ-cells, and is therefore called blastogenetic; the other acts indirectly through the body cells, and is therefore designated somatogenetic. Many of the blastogenetic factors bringing about structural changes in the delicate mechanism of germ-cells are entirely unknown, and are therefore designated as fortuitous. Many other causes, such as poisons in solution in the fluids that bathe them, can readily enough be appreciated.

Blastogenetic Factors. It has been demonstrated that various chemical substances, such as chloroform, morphia, chloral, etc., have a pronounced influence upon the vital activities of cells. It is well known that microscopic unicellular plants constitute the essential part of yeast. These little cells have the power of causing fermentation in solutions of grape sugar by which alcohol and carbon dioxide are formed, the latter being a gas and escaping as bubbles. If chloroform or ether be added to the solution of sugar, before adding the yeast, no fermentation takes place, for the yeast-cells are paralyzed. But when the yeast is separated from the chloroform solution and rinsed with distilled water, it soon regains the power of causing fermentation in pure solutions of sugar.

Ova and Spermatozoids are subject to the action of drugs in a similar manner. If actively motile spermatozoids of a sea-urchin be placed in a one-half of one per cent. solution of chloral in sea water, it will be found that after five minutes their action will be completely arrested. These motions can soon be restored if the chloral solution be sufficiently diluted with pure sea water. These temporarily paralyzed spermatozoids, when completely recovered, will unite as quickly with ova as fresh spermatozoids. When spermatozoids are kept for half an hour in the chloral solution, a more decided paralysis will be observed, which persists for some time after the removal of the poisonous agent. A few minutes elapse before some of the spermatozoids exhibit feeble movements which finally become active. Even when placed near ova, it is some time before they fertilize them, although several may attach themselves to the egg’s surface. But, finally, fertilization does take place by the penetration of one spermatozoid, and the egg normally develops.

In like manner, if ova are subjected to chloral solution of varying strength, they also are influenced in a marked degree; for, when fertilized, they develop in an abnormal manner. Ordinarily, normal ova are fertilized by one spermatozoid. If fertilized by two or more, they become diseased, and develop pathologically. The chloral solution favors this fertilization by several spermatozoids. The stronger the solution of chloral, the larger the number of spermatozoids that fertilize the ovum. Experiment and observation show that the behavior of the nuclear hereditary mass is modified, during mitosis, by the chloral and other solutions. It is thus seen that the germ-cells of the lower animals can be profoundly modified by various substances.

Equally true is it that the man or woman who makes use of such drugs as alcohol, opium, chloral, and such like, in an intemperate manner, contains these poisons in solution in the blood, circulating to every part of the body, and thus bathing and profoundly influencing the germ-cells. In consequence of this fact an acquired and habitual intemperance will seldom fail to leave its impress upon one or more of the offspring, either like the original vice or one very closely allied to it. Intemperate people not only profoundly impair the health, the intelligence, and the morals of their offspring, by poisoning these delicate germ-cells, but they also transmit the fatal tendency to crave for the very substances that have acted as poisons on these germ-cells before and after fertilization. And one of the saddest features of this great medical truth is that the hereditary units which are concerned in transmitting these grave abnormal tendencies may lie dormant in the germs of one generation, to become active in those of the next; so that children of intemperate parents may lead honorable and temperate lives, and take every pains to rear, in turn, their own children in a wholesome and refining atmosphere, and yet these children of good environment may become intemperate through heredity, so that the sins of the grandparents may be visited, not on the children, but on the grandchildren.

These profound truths should lead all, and especially law-makers, to remember that “the man who inherits from his parents an impulsive or easily tempted nature and an inert will and judgment, and commits a crime under the influence of strong emotion, can no more be placed in the same category of responsibility with a man of more favorable constitution and temperament than can a man who steals a loaf under the pangs of starvation with a merchant who commits a forgery to afford him the means of prolonging a guilty career.”[3]

Not only do certain known poisons circulating in the blood, or other fluid that may bathe the germ-cells of living creatures, profoundly affect the germ-cells, but many other substances probably have great influence upon them. Certainly, the amount and character of the food have a very decided influence on them, as will be understood from the following facts.

According to Yung, who has experimented very extensively upon tadpoles, all tadpoles pass through a bisexual (hermaphroditic) stage, as is the case probably with most animals. During this tadpole phase external influences, and, more particularly, food, determine their fate as regards sex. In Yung’s experiments it was found that when tadpoles were left to themselves, the percentage of females was in the majority, the average being probably about 57 per cent. females and 43 per cent. males. In experimenting with three broods, those fed on beef gave 78 per cent. females; those fed on fish gave 80 per cent. females; and those fed on the highly nutritious flesh of frogs gave 92 per cent. females.

In Mrs. Treat’s interesting experiments on moths and butterflies, it was observed that if caterpillars were confined and starved before they entered the chrysalis state, the resultant moths or butterflies were males, but others of the same brood that had been highly nourished came out females.

The study of bees illustrates the same conclusions. It is well known that in a beehive there are three kinds of inmates, as the queen, the drones, and the workers,—the last-mentioned being females whose reproductive organs are imperfectly developed. It is believed that the eggs that give rise to queens and workers are fertilized and developed normally. But it is a very curious fact that the eggs which develop into drones do so without fertilization (parthenogenesis). What factor or factors decide the destiny of the two former, determining whether a given ovum will develop into a queen, and thus be the possible mother of a new generation, or stay at the lower grade of a working, non-fertile female? These factors are the quality and quantity of the food. An abundance of what is called royal food causes the development of the larva in such a way that the queen with her reproductive organs is formed. If a larva on the road to develop into a worker (non-fertile female) “receive by chance some crumbs from the royal superfluity,” it is found that the reproductive organs may develop to such an extent that workers partially fertile may be formed. A worker larva may, by this royal food, be intentionally reared into a queen bee.

It is thus seen how profoundly the germ-cells, in their growth, may be affected and made to vary by such a blastogenetic factor as food.

Somatogenetic Factors. As to somatogenetic factors—granting that structural changes in the body (body-cells) of an animal or plant can profoundly influence in some way the germ-cells, and that, therefore, acquired characters can be transmitted—they are many and well defined. Some of them are the habitat of an animal or plant, the temperature, climate, air, food, soil, water, structures in use or disuse (so-called “Use” and “Disuse”), etc.

The following brief descriptions will enable the reader to understand that change in the surroundings (environment) of a living creature may cause its body (body-cells) to vary.

A certain species of snail was introduced into Lexington, Virginia, a few years ago from Europe. In its new habitat it varied very much. One hundred and twenty-five varieties have been discovered there, sixty-seven of which are new and unknown in Europe, the native home of the species.

The common ringed snake, when living in its natural habitat, deposits eggs in the sand, which are hatched by the heat of the sun; but when this snake is confined in a cage in which no sand is strewn, it gives birth to little living snakes.

In experimenting on moths it has been found that the variations of temperature to which the pupæ, and probably also the larvæ, are subjected, tend to bring about very pronounced differences in the moths. Cold has a tendency to develop a darker hue in the perfect insect.

English dogs when taken to hot climates, like that of India, are known to degenerate in a few generations. It is well known how climate affects the hairiness of animals. When greyhounds are taken to the uplands of Mexico they are unable to course on account of the rarity of the air.

In 1870 a number of pupæ of a certain species of moth (Saturnia) were taken from Texas to Switzerland. After passing the winter there, the pupæ emerged from their cocoons as moths, and resembled the Texan species entirely. The young of these moths were fed on the leaves of a plant different from that the moths in Texas feed on, and they developed into moths so different in form and color-pattern from their parents that entomologists classified them as a distinct species.

We have seen how certain foods affect the germ-cells and act as blastogenetic factors; the preceding case and the following show how certain foods act as somatogenetic factors and modify the body-cells. If the bullfinch be fed on hemp seed, its color is changed to black; if the canary be fed on cayenne, its plumage becomes darker; if the common green Amazonian parrot be fed on the fat of siluroid fishes, it assumes a beautiful variegation of red and yellow.

The character of the soil has a marked influence in inducing somatogenetic variations. In France an experimenter collected seed from the wild radish and sowed one lot in heavy soil in the country, while another lot was sown by him in the dry, light soil near the Museum of Natural History in Paris. The radish “roots” grown in these two places presented marked differences in color and form. Those grown in Paris were either of a rose or white color and elongated; while those from the country were violet, dark-brown or nearly black in color, and more rounded than the former.

In the summer of 1847 Professor Buckman gathered seed from wild parsnips, and sowed them in the spring of 1848 under changed conditions of life. Most of the plants grown from these seeds were like the wild parsnips, but some of them developed the light-green color and hairless, smooth appearance characteristic of the cultivated plant. The roots also were found to be more fleshy than those of the wild variety.

Peas and squashes, when grown in different soils, often show remarkable variations.

There is one species of shrimp that inhabits brackish water, and another that lives in water which is much more salt. These crustaceans differ from one another in the character of the spines they bear and in the form of the tail-lobes. They have been regarded as distinct species, and yet either of them can be transformed into the other in the course of a few generations, by gradually altering the saline conditions of the water.

For a long while the siredon and amblystoma were regarded as being distinct genera of amphibians. Siredon was looked upon as a permanent gill-breather, while amblystoma passes through a metamorphosis and becomes a permanent lung-breather. It is now known that the former can change into the latter. If there is plenty of water the siredon remains indefinitely a gill-breather and reproduces freely; but when the water dries up it changes into the lung-breathing amblystoma. These two cases illustrate very well the power of environment to modify the development of organic forms.

As to “use” and “disuse”: It can readily be observed that exercise increases the size of muscles; that by steady application the capacity for thinking can be developed; that the oarsman’s constant use of his hands leads to the hardening and thickening of the cuticle; that the arm of the blacksmith and the legs of the mountaineer are much enlarged, etc.

When an organ is exercised properly, there is an increased blood supply to it, and, consequently, stimulated nutrition and growth in various parts, such as in the muscular, nervous or other tissues.

When an organ is disused there is diminished blood supply, and, consequently, diminished growth and functional capacity. In man it is known that certain activities, such as coal-heaving, shoemaking, etc., produce recognizable effects upon the muscular system, the skeleton, and other parts of the body.

The peculiar habits of a tribe, such as tree-climbing among those natives of the interior of New Guinea, who build their houses in the upper limbs of lofty trees, modify the body in ways that are readily recognizable.

After considering many facts in connection with the brains of rabbits, Darwin announced that this most complicated and important organ in an animal is subject to the law of decrease in size from disuse. We have very interesting illustrations of the effects of “use” and “disuse” in causing somatogenetic variations, in the differences between domestic ducks and the wild ones from which they have been undoubtedly derived. The wild duck, which must constantly be on the alert for enemies, and uses its wings so much more extensively and its legs comparatively less than the domestic duck, is a much more intelligent fowl than the stupid, well-protected domestic one. The wings of the wild duck are stronger and its legs shorter than those of the barnyard duck. It has been shown that in the wild duck the brain is nearly twice as heavy in proportion to the body as it is in the comparatively imbecile domestic duck.

Many other useful illustrations of disuse, such as the cattle and goats in India, that have dependent ears; also cats in China, and horses in parts of Russia, whose ears are dependent, could be referred to. Use and disuse are included among the factors of environment, because by those terms we mean certain groups of body-cells that are functionally active or inactive; for body-cells on any theory of modified pangenesis constitute an exceedingly important environment of the germ-cells.

The surrounding conditions (environment) of an animal or plant having the power to cause variations in the living creature by affecting its germ-cells or its body-cells, the environment may be spoken of as blastogenetic and somatogenetic.

Whether it is a fact or not that somatic variations can induce corresponding variations in the germ-cells, and thus be transmitted by heredity, it is certainly true that all heritages must come through the germ-cells. For this reason, it is clearly seen that so far as evolution is concerned the germ-cells are the factors of fundamental importance in organisms. Therefore, we may repeat that environment is the sum-total of the conditions of life that affect the germ-cells directly or indirectly.

ACQUIRED CHARACTERS.

All heritages, then, are derived directly through the germ cells. Can there be any heritages indirectly from the somatic cells through the germ-cells, as has hitherto been assumed? In other words, can acquired characteristics be transmitted to the offspring? This question has given origin to the battle royal that is still going on between opposing schools of biology. The contending parties have appealed to such biological evidence as is furnished by a study of use-inheritance, reflex and instinctive actions in animals, etc., and to such experimental evidence as the induction of traumatic epilepsy in guinea pigs, a change in the shape of the ear by cutting the cervical sympathetic nerve, protrusion of the eyeball through injury to the restiform body of the brain, and such like, noting the effects on the offspring, and have drawn very different conclusions.

As to the transmission or non-transmission of acquired characters, some have maintained that only germinal variations are transmitted (because they believe the germ cells are insulated from the body cells, and therefore from somatic influences). For instance, Ziegler, in his work on General Pathology, says: “If a disease, such as nearsightedness, is the product of a special inherited predisposition, plus the effect of harmful influences which have acted upon the body during life, only that part can be transmitted which was received by inheritance, but not that part which was derived from external influences.” In other words, there is no transmission of acquired character. In this belief it will be observed that he follows Weismann.

On the contrary, other investigators, like Darwin and Spencer, teach that somatic variations—the plus element in Ziegler’s illustration of nearsightedness—do influence the germ-cells (through some such agency as Darwin’s theory of pangenesis suggests), and that, therefore, acquired characters can be transmitted. The question is one of fundamental importance, and yet no crucial experiment has been devised or fact observed which can compel the correct answer. The evidence seems to favor the view that acquired characters can be transmitted.

The theories as to the transmission or non-transmission of acquired characters may be better understood by reference to schemes No. 1, 2 and 3. Scheme No. 1 represents the theory of Pangenesis, which teaches that reproductive cells are not formed from pre-existing reproductive cells, but by the body cells themselves. Darwin taught that all the cells of the body, such as skeletal-cells, muscle-cells, nerve-cells, and so on, are continually giving off infinitely small cell germs or gemmules, which have the power of growing and forming cells exactly like themselves. These gemmules have a great affinity for one another, and, circulating in the blood in countless numbers, they finally come together in the reproductive glands and form the reproductive cells. On this theory the fact of the transmission of acquired characters can readily be appreciated, and it can easily be understood how the parent molds the child. Suppose, for instance, that the parent, by exercise, has become a skillful athlete. In him certain muscles have become greatly developed and strengthened. During all the time of the exercise of these muscles, the modifying muscle cells have been continually giving off to the blood modified gemmules, which collect in the reproductive cells and make it possible for the offspring to develop into an athlete because the modified gemmules develop into modified muscles like those of the athlete.

Scheme 1 shows the absence of any arrow like those shown in schemes 2 and 3, directly connecting germ-cell with germ-cell; this means that in this theory there is no continuity of the germ-cells. But arrows are seen extending from the various body-cells (skeletal, glandular, etc.) to the germ-cell; this means that the germ-cells are formed by influences or gemmules emanating from the various body cells.

Scheme 2 teaches that a germ-cell (when fertilized, of course) can produce many cells, some of which differentiate, finally, into skeletal cells, some into glandular, some into muscle and nerve cells, and some into new germ-cells; so that an animal or plant, I, is formed. In like manner a germ-cell of animal, I, can give rise to the germ and body cells of animal, II, and so on indefinitely. This scheme shows that there is a direct continuity of the germ-cells; and it also shows that the germ-cells are entirely insulated, as it were, from the body-cells (skeletal, glandular, etc.), inasmuch as no influences (arrows) extend from the body-cells to the germ-cells. This means that the transmission of acquired characters, bodily, mental, moral, etc., is impossible. It means, in other words, that none of the advantages gained by a parent in the course of his life can be handed on to his offspring by heredity. There are many biologists and pathologists who teach this theory as the correct one.

The majority of biologists accept the theory illustrated in Scheme 3. This is the theory of modified Pangenesis, which teaches that there is a direct continuity of the germ-cells and that these germ-cells are not insulated from the body-cells, but that the latter, when modified as the result of experience, can send off influences that correspondingly modify the germ-cells; so that the latter, when developing into a new individual, may cause the same body variations that exist in the parent. In short, this scheme illustrates not only that there is a germinal inheritance, but also an inheritance of acquired characters. In this Scheme 3, the oblique arrows show that germ-cells produce other germ cells; the perpendicular arrows show that the germ-cells are modified by influences that proceed from the body cells.

Germinal characteristics are transmitted with vastly greater amplitude and swiftness than merely body (acquired) characteristics. If, for instance, a man were born with that physical constitution that makes with ease a first-class pianist out of him, his sons may easily, through heredity, be first-class pianists. But if a man be born without such a congenital tendency and has by constant labor and practice so developed the muscles of his forearm, his nerves, his brain, etc., that he becomes a very good pianist (acquired characters); and, further, if his male descendants for thousands of generations, in succession, have become very good pianists by constant practice, we may expect that the sons of these last generations may obtain a congenital tendency to become first-class pianists quite easily. The constant improvement, by practice, of groups of body-cells (muscle-cells, nerve-cells, etc.) for generations, has, in each generation, tended to so correspondingly modify the germ-cells that they have acquired the power to develop into men who may become very good pianists with very little practice. This illustrates that there may be a continuous summation of feeble germ-cell variations that have been induced by prolonged influences emanating from somatic variations, so that, in the course of many generations, robust acquired characters may ultimately be translated into strong congenital characters (Scheme 3).

Scheme 1. Illustrating the theory of Pangenesis. Here the germ-cell (a) develops into the body-cells, e, e, e, e, of animal I, as indicated by the oblique arrows, but not into any germ-cells, as indicated by absence of arrow between germ-cell (a) and germ-cells (b). The germ-cells (b) in animal I are formed by the aggregation of infinite numbers of gemmules from the various groups of body-cells, e, e, e, e, as indicated by the perpendicular arrows. The germ-cell (a) transmits germinal heritages to the body-cells e, e, e, e; these body-cells transmit the heritages to the germ-cells (b) by means of the gemmules. If the body-cells are modified in any way, correspondingly modified gemmules are sent to the germ-cells (b), and these germ-cells are modified and thus transmit acquired characters to animal II, and so on.

Scheme 2. Illustrating the theory of Continuity of the Germ-Cells; pure germinal inheritance; and the non-transmissibility of acquired characters. The germ-cells are insulated from the body cells. The germ-cell (a) develops into the body-cells, e, e, e, e, and the germ-cells (b), in animal I. The body-cells, e, e, e, e, do not influence in any way the germ-cells (b), as indicated by the absence of perpendicular arrows. The germ-cells (b) get all their heritages from the antecedent germ-cell (a), as indicated by the oblique arrow from (a) to (b). All heritages are purely through the germ-cells. The same with the animals II and III. Germ-cells (a), (b), (c), (d), are connected together by obliquely placed arrows, indicating the continuity of the germ-cells.

Scheme 3. Illustrating the theory of Continuity with Modified Pangenesis. A germ-cell (a) develops into the body-cells, e, e, e, e, and the germ-cells (b) of animal I. The germ-cells (b) get their heritages directly from the germ-cell (a), as indicated by the long, obliquely-situated arrow (continuity of the germ-cells). The germ-cells (b) are, moreover, modified by influences extending from the body-cells, e, e, e, e, as indicated by the perpendicular arrows. A modified germ-cell (b) can develop into a modified animal II, and the body-cells of this animal can influence and modify the germ-cells (c); and so on, indefinitely. The perpendicular arrows indicate that acquired characters are transmitted, and that, too, through the germ-cells.

Professor Morgan, of England, has advanced the ingenious theory, which may reconcile the above-mentioned antagonistic views, that somatic variations, in the direction of adaptation, pave the way for germinal variations, so that, while somatic modifications as such are not inherited, they are yet the favoring conditions under which germinal variations are preserved by the great principle of natural selection. If this is true, as we think it is, then we can safely state that each man in his totality is the resultant of two great factors—heredity and environment, the latter including not only food, water, climate, occupation, etc., but also the character of the civilization, the state of morals in society, the ideals and examples most frequently seen, etc., etc.

Heredity brings down to him the streams of tendency of former generations, often of a healthy and beneficent character, but also often surcharged with lust and passion, and reeking with disease.

Environment is the coöperating and, to us, vitally important factor, inasmuch as it may supplement and thus reënforce the hereditary tendencies, whether good or bad; or it may even tend to turn them into new channels, correcting the evil or vitiating the good.

Man is not simply a creature of the present, but profoundly a product of the past. Bodily structure, moral and intellectual tendencies, disease, vices, and virtues are all in the marvelous stream of heritage that comes to him from the past. “Diseases that no facts in the individual life can account for point gaunt fingers of blame from one generation to another. Not a murderer is hung, not a daughter starts on the downward way, but a great company, like those who were present at the stoning of Stephen, stand by inaugurating and consenting to the ruin.”[4]

Truly has it been said that the past is at work in the present, its powers reaching down through the ages, to all the race, largely molding every human life, touching and influencing every individual’s thought and will, and, more than any other force, coloring history.

Studies in heredity illustrate most luridly that the continuity of the human race is a terrible but remorseless reality.

If the ignorance and the perverted pleasures of one generation may produce the vices and the crimes and the diseases of another, a question of tremendous import arises: Is heredity as potent in the direction of virtue and health as of vice and disease? At the first look one is almost tempted to answer Nay! for the most striking examples of heredity seem to be in the direction of evil. But this is perfectly natural. Decay is always more rapid than growth. A cherry rots much more quickly than it ripens. Vice and disease spread much more quickly and widely than virtue and health. But all history and all social and medical science teach that vice and disease carry within themselves the seeds of decay, and virtue and health the seeds of endurance and growth.

Through the great Darwinian principle of natural selection, or survival of the fittest, vice and disease will become less and less predominant, and virtue and hygienic constitutions more and more disseminated.

As influencing a man’s life and character,[5] which is the stronger factor, heredity or environment? Fatalism or choice? In our opinion, as the result of long study and reading, where we have an average man of “mens sana in corpore sano,” environment will be the stronger factor whether for good or for evil—that is, in men in general, who have no organic defect, such as insanity or idiocy, and allied affections, the stronger force is environment; but in those having such defect, heredity is the controlling power, and, we may add, the destroying power.

It must be recalled, though, that the average man with a “sound mind in a sound body,” in his development to his present estate, has become possessed of a vast aggregate of diverse heritages, of varying age, strength and dignity. Some of them are so old and strong that they seem to be cast in unyielding molds, while others are so weak and recent that they fluctuate with every passing circumstance. The most dignified and important of all his heritages is that of rational volition. It is the play of this volition upon many of his other heritages that gives him the power of selecting, to a limited extent, his environment.

Every man is born into the world with a certain physical constitution, and, therefore, with a given temperament; with certain passions; with the power of judgment; and with a certain strength of will. If the power of his will be not equal to the strength of his passions, the latter will surely predominate and will display him as the slave of heredity. If he has such an organization of his nervous system that his volition is superior to his passions, he will be none the less the servant of heredity, though a being now possessed of the power of Free-Will.

Man is, to a far greater degree than is ordinarily realized, the servant of heredity. It seems to us an incontrovertible fact that every living creature, at any given moment, is swayed infinitely more by the totality of its heritages than by its environment. No one can possibly deny this so far as plants and most animals are concerned. Nor, if one look below the surface, can it be denied of the higher animals and of man. Happily, the average man, with his present constitution, has his diverse heritages so proportioned that we may repeat that his life and character (in customs, morals, and religion) are vastly more influenced by environment than by heredity.

The standards for estimating the life and character of men, namely, human customs, morals, and religions, are such recent acquisitions, geologically speaking, that they have, as yet, very slightly if at all influenced the germ-cells. They are acquired (somatic) characteristics, and not congenital (germinal) qualities. They are preëminently the creations of environment. If the infants of a Catholic family which is descended from a long line of Catholic ancestors were to be placed and retained in a purely Mohammedan environment, heredity would carry no Christian customs, morals or religion into that environment, but, on the contrary, the Mohammedan surroundings would instill new customs, different ethical ideas, and a different religion. This illustrates how very feebly indeed are germ-cells correspondingly impressed by pure acquired characters. It is almost certain that the translation of somatic changes into germinal changes is appallingly slow. As far, then, as social customs, morals, and religion are concerned, the average man is, in our opinion, infinitely more the creature of nurture than of nature. But, as far as his temperament, his emotional nature, his judgment, his strength of will, in short, his physical and therefore his mental constitution, are concerned, he is almost absolutely the creature of heredity. The equilibrium of qualities or heritages in the average man, resident in a given, stable community, is in harmony with the average customs, ethical ideas, and religious beliefs of that community. But in all stable communities there are men whose resultant of heritages, some in one direction and some in another, places them out of harmony with the average of their social environment, and they are looked upon, some as idiots, some as geniuses, some as criminals, and others as saints, and so on. So that again we may say that a man’s character in a community is the resultant of an hereditary physical constitution, and his environment. Some men may inherit such a physical constitution that in spite of the best environment they are much debased below the average man; others may possess such heritages that, notwithstanding adverse circumstances, they reach a level of character much above the average man. And there are all gradations between the two extremes.

SECTION III.
UNSTABLE ENVIRONMENT.

UNSTABLE ENVIRONMENT.

Where living creatures are in harmony with their surroundings,—where, in other words, they are adapted to their environment,—and where, further, this environment is apparently in a state of equilibrium; there we find the fewest and least marked variations in the living creatures. To the casual observer the face of nature maintains the same guise from year to year. The earth seems solid and unyielding; the mountains appear to be everlasting; the restless waters of rivers and brooks seemingly move and throb in the same channel; the tides ebb and flow in apparently unchanging ocean beds; the birds and flowers and woodlands look alike from year to year; and all the varied phenomena of nature appear completed and permanent, as if the present world were constructed in an unyielding mold.

But nothing is fixed and rigid in nature. The earth itself travels rapidly through space and brings in due season spring, summer, autumn and winter; revolves upon its axis and alternates the starry night with sunshine; and periodically changes its orbit so that at one time the northern pole has a temperate climate where water lilies may grow, and at another period presents an arctic climate with impassable barriers of ice. Ice and frost and other forces are breaking up the rocks of mountains, making larger and smaller fragments and even powder; the rains descend and the mountain brooks are swollen to resistless torrents which carry the fragments and mud to the rivers, and these latter take the mud on to the ocean.

Thus, by degrees, the mountains, hills and all the earth are being eroded and the great bulk of the detritus carried by the rivers to the sea and deposited along the sea margins. Thus sedimentation goes along with erosion, and gradually marginal sea bottoms of immense thickness are formed, which will in time be consolidated into rocks and uplifted as dry land. The ceaseless grinding of waves and tides erodes the coast line and adds débris to the marginal sea bottom. The finest sediment is carried out by the tides so far as to reach the ocean currents, and thus is strewn broadcast over portions of deep-sea bottom, and will also in due time be consolidated into rock.

Myriads of animals that form calcareous shells live and die in the ocean. The shells of the dead animals are falling like a perpetual shower on certain ocean bottoms, year after year, so that immense accumulations of calcareous substances occur there, which will also in time be consolidated into limestone rocks, and uplifted as dry land.

Deltas are forming at present at the mouths of certain rivers, and estuaries at others. Lands are now gradually emerging from the sea in some places and at others sinking into the sea. The entire coast of Scandinavia, both on the Baltic and Atlantic sides, is rising out of the sea, and has been doing so for a long time. It is rising at the rate of more than two feet in a hundred years. During an immense period of time there has been a gradual elevation of all the southern part of the South American continent. Sometimes a large elevation takes place rapidly. In 1822, and again in 1835, the southwest coast of South America, after severe earthquakes, was elevated several feet along a distance of several hundred miles. It is known that the coast of Greenland, for five hundred miles, is subsiding. From a study of coral barriers and atolls it is believed that an area of the mid-Pacific sea bottom covering over ten million square miles is sinking and has been doing so for a long time.

The foregoing facts tend to illustrate the truth that nothing is permanent in the environment of living creatures at present. All surroundings are perpetually changing, though apparently ever so slowly. The changes that are now going on were also taking place yesterday, last week, last century, last æon, and so on throughout geologic time. Gradual oscillations of the earth’s crust on a grand scale and affecting whole continents, but usually so slowly as to escape popular observation, have been taking place ceaselessly through inconceivable ages. These oscillations have produced all the great inequalities of the earth’s surface, such as ocean basins, continents and mountain chains. The oscillations are probably due to the slow cooling and unequal shrinking of the whole earth which has been progressive through all geologic time.

The state of the contest between the eroding and the uplifting agencies of the world at any time determines the height of mountains and continents, the depths of seas, the distribution of land and water, for that period.

Fig. 11.—Archæan North America. The white part of the drawing indicates the emerged land; the dark shading indicates the submerged land covered by a shallow sea; the light shading indicates the deep sea.

From Shaler’s First Book in Geology. By courtesy of the publishers, D. C. Heath & Co.

Knowing that the present physical agencies at work on the globe have been acting through long ages,[6] it can readily be appreciated how small effects have been accumulated and low elevations, for instance, have become immense, high mountain ranges. The growing mountain ranges alter the climate and the meteorological conditions. The rainfall on one side differs from that on the other. The temperature varies with the altitude, and so on.

Although continents have gradually and steadily grown from the earliest times, there have been many local alterations of land and sea. Marginal sea bottoms have become great mountain ranges. Islands have appeared and sunk from view. Lakes have been gradually converted into solid land or into peat bogs. Fresh water bodies have become brackish. Dry lands have become marshes, and forests have been buried beneath the waves. Geologic changes have caused great alterations in climate at given times and in given areas.

These statements may be illustrated and emphasized by a brief reference to the development of the Continent of North America. This Continent has grown from comparatively a small beginning to its present great proportions. In doing so it has passed through eras of stupendous duration. These eras in the order of their occurrence are as follows:

(1) Archæan era; (2) Palæozoic era (subdivided into Cambrian, Silurian, Devonian and Carboniferous periods); (3) Mesozoic era (subdivided into Triassic, Jurassic and Cretaceous periods); (4) Cenozoic era (subdivided into Tertiary and Quaternary periods); the Tertiary is subdivided into Eocene, Miocene and Pliocene epochs; the Quaternary is subdivided into Glacial, Champlain and Terrace epochs; (5) Psychozoic era, or recent epoch.

The physical geography of the continent at the close of that early geologic era known as the Archæan is shown on the map ([Fig. 11]). At this time the vast portion of the continent, whose outlines nevertheless existed, was submerged under a shallow sea, as indicated by the dark shading on the figure. The white V-shaped mass, starting just above the site of the great lakes and extending on the one hand in a northeasterly direction to Labrador, and on the other in a northwesterly direction to the Arctic Ocean, is the emerged land of this Archæan time. Smaller masses of Archæan land are also seen at the site of the Blue Ridge Mountains in the east and at that of the Rocky Mountains in the west. Around these lands as a nucleus the North American Continent has been built. Therefore, at the close of the Archæan or beginning of the next, or Palæozoic era, the whole interior portion of the continent was covered by a shallow sea which beat against the Canadian Archæan land on the north, the Blue Ridge Archæan land on the east, and the Rocky Mountain Archæan land on the west. This shallow sea is known as the Palæozoic Sea. Throughout the vast ages of the Palæozoic era, immense sediments were being deposited along the marginal sea bottoms. The deposition of these sediments was simultaneous with a further sinking of the submerged continent, so that the shallowness of the Palæozoic Sea was maintained; finally, the uplifting forces predominated, and the submerged land along the margins of the Canadian Archæan appeared as dry land, and thus increased the area of the infant continent. During all this period there was a steady and slow growth of the land southward from the Canadian Archæan, so that towards its close the visible continent had increased nearly, though not exactly, to the proportions attained in a still later (Cretaceous) period ([Fig. 12]).

Fig. 12.—Cretaceous North America. The white portion of the figure indicates emerged land—the growing continent.

From Shaler’s First Book in Geology. By courtesy of the publishers, D. C. Heath & Co.

At the close of the Palæozoic era the slow, steady changes that had been going on were replaced by more rapid and comparatively revolutionary changes, which caused great alterations in the physical geography and climate. Hitherto the continent had been comparatively low. Now the vast sedimentary accumulations constituting the marginal sea bottom of the eastern portion of the Palæozoic Sea, which had been accumulating through all Palæozoic time, were uplifted into the great Appalachian chain of mountains.

During the earlier ages (Silurian and Devonian) of this Palæozoic era, the place of the Appalachian chain of mountains was marginal sea bottom; but during the later ages (Carboniferous) it was, through repeated oscillations, in an uncertain state, being sometimes swamp land, sometimes covered with river sediment, and sometimes covered by the sea. It was during this Carboniferous age that the great coal measures were formed; at this time also the climate was probably very uniform, warm and moist, loaded with carbonic acid gas and deficient in oxygen. This period was undoubtedly a paradise for the great coal-forming plants, but was very unsuitable for the hot-blooded air-breathing animals, such as mammals and birds, none of which existed at that period. Throughout all geological time the excessive amount of moisture in the air has been gradually removed by the growth of continents in size and height; also the superabundant carbon dioxide in the atmosphere has been removed in many ways, especially by the plants in the coal period appropriating the carbon. Many ages later, at the close of the Jurassic period, the Sierra Nevada range of mountains was uplifted. Up to this time the site of these mountains was a marginal sea bottom receiving vast amounts of sediment, and the Pacific coast-line was east of the site of the Sierra range. Naturally vast changes in physical geography and climate occurred in consequence. During these and the following Cretaceous ages that the continent was growing, the great interior Palæozoic sea and what may be called the Gulf of Mexico were more and more restricted, as shown in the map of North America in the Cretaceous period of its growth ([Fig. 12]). This great inland sea, separating the continent into an eastern and western portion, is now called Cretaceous Sea instead of Palæozoic. This Cretaceous Sea covered the whole plains and plateau region of the continent, and extended from the Gulf of Mexico to the Arctic Ocean. At the end of the Cretaceous period of the continent this sea was obliterated by the gradual upheaval of this region and replaced by great lakes. At the same time the western marginal bottom of the sea was uplifted into the Wahsatch range of mountains; also at this time a line of islands in the Cretaceous Sea was uplifted into the Colorado mountains. All these events were entailing tremendous changes in physical geography and climate. [Fig. 13] is a representation of the map of North America in the early Tertiary period, the time succeeding the Cretaceous period. In this period, the continent continuing to uplift, the lakes that occupied the site of the Cretaceous Sea are obliterated; the Coast Range mountains of California and Oregon are uplifted from marginal sea bottom ([Fig. 13], dark shading); the Atlantic and Gulf borders are extended (dark shading), so that at the close of the Tertiary period the North American continent had attained its present form, except the southern portion of Florida and its keys. Since then the latter have grown and are still growing.

Fig. 13.—Early Tertiary North America.

From Shaler’s First Book in Geology. By courtesy of the publishers, D. C. Heath & Co.

A later epoch still in the history of the globe is known as the Quaternary period, the period that immediately preceded our present epoch. The great features of this period, which is divided into Glacial, Champlain and Terrace epochs, are the wide-spread oscillations of the earth’s crust in high latitudes towards the north and south poles, attended with great changes of climate from temperateness to extreme cold.

The Glacial epoch was characterized by upward crust movements, the land becoming over one thousand feet higher than at present. The land was covered with ice, and an arctic severity of climate extended almost to the Gulf of Mexico. The Champlain epoch was characterized by a downward movement of the coast until it became five hundred feet or more below the present level, so that many lower portions of the continent became covered with sea. At this time there was a moderation of the temperature, a melting of the vast sheets of ice, and consequently a flooding of rivers and lakes, with many icebergs floating in them. The last or Terrace epoch of the Quaternary period was characterized by a crust movement up to the present condition of things.

What is true of the instability of the North American Continent is true of all the continents of the globe. They have all grown from small beginnings to their present huge proportions, and are now undergoing slow but irresistible changes. When these facts are held in mind, one may form a faint conception of the colossal changes that have taken place throughout the sweep of bygone ages. Environment means a complexity of conditions almost infinite in their number and character, and almost infinite in their variations.

SECTION IV.
TRANSMUTATION OF LIVING FORMS.

TRANSMUTATION OF LIVING FORMS.

It ought now to be understood that not only is the present environment changing, but also that it has been changing from the earliest geologic times. What, then, is to be said about the living creatures that have existed in the changing environment during all these geologic ages? Have they been rigid, unyielding forms? By no means! We know that they can be modified by altering the conditions at present; and a study of the fossils in the rock formations of the different ages of the world shows conclusively that animals and plants have altered in the past with the changing environment. The living creatures in the Silurian ages differ from those in the succeeding Devonian ages, and these latter differ from those in the still later Carboniferous ages; and so on, to the present. Changing physical geography and climate are associated with changing forms in animal and plant life. The growing amplitude and complexity of a continent are associated with increasing complexity and specialization of its living forms. Just as the North American Continent of the Tertiary period differs from that of the Silurian ages, so also do the animal and plant forms of the Tertiary period differ from those of the Silurian ages. Just as there has been a continuity in the growth of the Silurian continent to that of the Tertiary ages, and the present, so, also, there has been a continuity of living creatures from Silurian to Tertiary and present times. Changing conditions of life have compelled modifications in living forms, and those creatures that were unable to adapt themselves to the altering conditions of life have perished, while those that did adapt themselves, through useful variations, lived and progressed in organization.