Transcriber's note:
Genus names are not consistenly italicized in the original book. These have been corrected for consistency according to the modern usage except in the Index where they are consistently printed in regular fontface.

Life in the Silurian Age

THE CHAIN OF LIFE

IN

GEOLOGICAL TIME

A SKETCH OF THE ORIGIN AND SUCCESSION OF
ANIMALS AND PLANTS

BY

SIR J. WILLIAM DAWSON

C.M.G., LL.D., F.R.S., F.G.S., Etc.

AUTHOR OF
“ACADIAN GEOLOGY,” “THE STORY OF THE EARTH,” “EGYPT AND SYRIA; THEIR
PHYSICAL FEATURES IN RELATION TO BIBLE HISTORY,” ETC.

THIRD AND REVISED EDITION

WITH NUMEROUS ILLUSTRATIONS

THE RELIGIOUS TRACT SOCIETY,

56 Paternoster Row; 65 St. Paul’s Churchyard;
and 164 Piccadilly

1888

Richard Clay and Sons.
LONDON AND BUNGAY.

PREFACE.

Questions as to the origin and history of life are not at the present time answered by mere philosophical speculation and poetical imagining. Such solutions of these questions as science can profess to have obtained are based on vast accumulations of facts respecting the remains of animals and plants preserved in the rocky beds of the earth’s crust, which have been successively accumulated in the course of its long geological history. These facts undoubtedly afford the means of attaining to very certain conclusions on many points relating to the history of life on the earth. But, on the other hand, they have furnished the material for hypotheses which, though confidently affirmed to be indisputable, have no real foundation in nature, and are indirectly subversive of some of the most sacred beliefs of mankind.

In these circumstances it is most desirable that those who are not specialists in such matters should be in a position to judge for themselves; and it does not appear impossible in the actual state of knowledge, to present, in terms intelligible to the general reader, such a view of the ascertained sequence of the forms of life as may serve at once to give exalted and elevating views of the great plan of creation, and to prevent the deceptions of pseudo-scientists from doing their evil work. Difficulties, no doubt, attend the attempt. They arise from the number and variety of the facts, from the uncertainties attending many important points, from the new views constantly opening up in the progress of discovery, and from the difficulty of presenting in an intelligible form the preliminary data in biology and geology necessary for the understanding of the questions in hand. In order, as far as possible, to obviate these difficulties, the plan adopted in this work has been to note the first known appearance of each leading type of life, and to follow its progress down to the present time or until it became extinct. This method is at least natural and historical, and has commended itself to the writer as giving a very clear comprehension of the actual state of our knowledge, and as presenting some aspects of the subject which may be novel and suggestive even to those who have studied it most deeply.

In selecting examples and illustrations, the writer has endeavoured to avoid, as far as possible, those already familiar to the general reader. He has carefully sought for the latest facts, while rejecting as unproved many things that are confidently asserted; and has endeavoured to avoid all that is irrelevant to the subject in hand, and to abstain from all technical terms not absolutely essential. In a work at once so wide in its scope, so popular in its character, and so limited in its dimensions, a certain amount of hostile criticism on the part of specialists is to be expected, some portion of it perhaps just, other portions arising from narrow prejudices due to limited lines of study. The writer is willing to receive such comments with attention and gratitude, but he would deprecate the misuse of them in the interest of those coteries which are at present engaged in the effort to torture nature into a confession of belief in the doctrines of a materialistic or agnostic philosophy.

The title of the work was suggested by that of Gaudry’s recent attractive book, Les Enchaînements du Monde animal. It seemed well fitted to express the connection and succession of forms of life, without implying their derivation from one another, while it reminds us that nature is not a fortuitously tangled skein, and that the links which connect man himself with the lowest and oldest creatures bind him also to the throne of the Eternal.

In the few years that have elapsed since the publication of the first edition of this work, great additions have been made to our knowledge of fossil animals and plants. Many new species have been described, and many new facts have been discovered, respecting species previously known. This rapid progress of discovery has, however, invalidated few of the statements made in the first edition, and has certainly established nothing against the general laws of the succession of life as stated in this work.

Perhaps the most interesting phase of recent discovery is the tracing back of certain forms of life to earlier periods of the earth’s geological history. Some of the most recent facts of this kind are the finding, by M. Charles Brongniart, of a fossil insect, allied to the Blattae or cockroaches, in the Silurian of Spain, that of true Scorpions in the Upper Silurian of Sweden by Lindström, and in the Upper Silurian of Scotland by Peach, who has also described fossil Millipedes from the Lower Devonian. The tendency of such discoveries is to carry farther back the origin of highly specialised forms of life, and thus to render less probable their origin by any process of gradual derivation.

Other discoveries serve to fill up blanks in our knowledge, and thus to render the geological record less imperfect. Of this kind is the close approximation now worked out in Western America between the end of the reign of the great Mesozoïc reptiles and the beginning of that of the mammals of the Tertiary—a great and abrupt revolution, effected apparently by a coup de main. I have myself had opportunity to show that a similarly sharp line separates that quaint old Mesozoïc flora of pines, cycads and ferns, which extends upward into the Lower Cretaceous, from the rich and luxuriant assemblage of broad-leaved trees of modern aspect, which takes its place in the middle part of the same formation.

It is not too much to say that these and similar discoveries, while they serve to bridge over gaps in the succession of organic beings, do not favour the theory of slow modification of types. They rather point to a law of rapid development of new forms under special conditions as yet unknown to science, and this accompanied with the extinction of older species. Recent discoveries also present many remarkable instances of the early introduction of highly specialised types, of higher forms preceding those that are lower in the same class, and of the persistence of certain types throughout geological time without any important change.

J. W. D.

McGill College.

CONTENTS.

LIST OF ILLUSTRATIONS.

Tabular View of Geological Periods and of Life-Epochs.

THE CHAIN OF LIFE.

CHAPTER I.

preliminary considerations as to the extent and sources of our knowledge.

t is of the nature of true science to take nothing on trust or on authority. Every fact must be established by accurate observation, experiment, or calculation. Every law and principle must rest on inductive argument. The apostolic motto, “Prove all things, hold fast that which is good,” is thoroughly scientific. It is true that the mere reader of popular science must often be content to take that on testimony which he cannot personally verify; but it is desirable that even the most cursory reader should fully comprehend the modes in which facts are ascertained and the reasons on which conclusions are based. Failing this, he loses all the benefit of his reading in so far as training is concerned, and cannot have full assurance of that which he believes. When, therefore, we speak of life-epochs, or of links in a chain of living beings, the question is at once raised—What evidence have we of the succession of such epochs? This question, with some accessory points, must engage our attention in the present chapter.

Geology as a practical science consists of three leading parts. The first and most elementary of these is the study of the different kinds of rocks which enter into the composition of those parts of the earth which are accessible to us, and which we are in the habit of calling the crust of the earth. This is the subject of Lithology, which is based on the knowledge of minerals, and has recently become a much more precise department of science than heretofore, owing to the successful employment of the microscope in the investigation of the minute structure and composition of rocks. The second is the study of the arrangement of the materials of the earth on the large scale, as beds, veins, and irregular masses; and inasmuch as the greater part of the rocks known to us in the earth’s crust are arranged in beds or strata, this department may be named Stratigraphy. A more general name sometimes employed is that of Petrography. The third division of geology relates to the remains of animals and plants buried in the rocks of the earth, and which have lived at the time when those rocks were in process of formation. These fossil remains introduce us to the history of life on the earth, and constitute the subject of Palæontology.

It is plain that in considering what may be learned as to epochs in the history of life we are chiefly concerned with the last of these divisions. The second may also be important as a means of determining the relative ages of the fossils. With the first we have comparatively little to do.

Previous to observation and inquiry, we might suppose that the kinds of animals and plants which now inhabit the earth are those which have always peopled it; but a very little study of fossils suffices to convince us that vast numbers of creatures once inhabitants of this world have become extinct, and can be known to us only by their remains buried in the earth. When we place this in connection with stratigraphical facts, we further find that these extinct species have succeeded each other at different times, so as to constitute successive dynasties of life. On the one hand, when we know the successive ages of fossil forms, these become to us, like medals or coins to the historian, evidences of periods in the earth’s history. On the other hand, we are obliged in the first instance to ascertain the ages of the medals themselves by their position in the successive strata which have been accumulated on the surface. The series of layers which explorers like Schliemann find on the site of an ancient city, and which hold the works of successive peoples who have inhabited the place, thus present on a small scale a faithful picture of the succession of beds and of forms of life on the great earth itself.

Our leading criterion for estimating the relative ages of rocks is the superposition of their beds on each other. The beds of sandstone, shale, limestone, and other rocks which constitute the earth’s crust have nearly all been deposited thereon by water, and originally in attitudes approaching to horizontality. Hence the bed that is the lower is the older of any two beds. Hence also, when any cutting or section reveals to us the succession of several beds, we know that fossil remains contained in the lower beds must be of older date.

We can scarcely walk by the side of a stream which has been cutting into its banks, or at the foot of a sea-cliff, or through a road-cutting, without observing illustrations of this. For instance, in the section represented in [Fig. 1], we see at the surface the vegetable soil, below this layers of gravel and sand, below this a bed of clay, and below this hard limestone. Of these beds a is the newest, d the oldest; and if, for example, we should find some marine shells in d, some freshwater shells in c, bones of land animals and flint arrowheads in b, and fragments of modern pottery in a, we should be able at once to assign their relative ages to these fossils, and to form some idea of the succession of conditions and of life which had occurred in the locality.

On a somewhat larger scale, we have in [Fig. 2] a section of the beds cut through by the great Fall of Niagara. All of these except that marked a are very ancient marine rocks, holding fossil shells and corals, but now forming part of the interior of a continent, and cut through by a fresh-water river.

Fig. 1.—Bank of stream or coast, showing stratification.

a, Vegetable soil. b, Gravel and sand. c, Clays. d, Limestone rock, slightly inclined.

Fig. 2.—Section at Niagara Falls, showing the strata cut through by the action of the Fall. Thickness of beds about 250 feet.

In deep mines and borings still more profound sections may be laid open, as in [Fig. 3], which represents the sequence of beds ascertained by boring with the diamond drill in search of rock salt near Goderich in Canada. Here we have a succession of 1,500 feet of beds, some of which must have been formed under very peculiar and exceptional conditions. The beds of rock salt and gypsum must have been formed by the drying up of sea-water in limited basins. Those of Dolomite imply precipitation of carbonate of lime and magnesia in the sea-bottom. The marls must have been formed largely by the driftage of sand and clay, while some of the limestone was produced by accumulation of corals and shells. Such deposits must not only have been successive, but must have required a long time for their formation.

Fig. 3.—Section obtained by boring with the diamond drill, near Goderich, Ontario, Canada, in the Salina series of the Upper Silurian. From a memoir by Dr. Hunt in the Report of the Geological Survey of Canada for 1876-7.

No. 1, Clay, gravel, and boulders—Post-pliocene.
Nos. 2, 4, 7, 9, 13, Dolomite or magnesian limestone, with layers of marl, limestone, and gypsum.
No. 3, Limestone with corals—Favosites, etc.
Nos. 5, 11, 15, 17, Marls with layers of Dolomite and anhydrous gypsum.
Nos. 6, 8, 10, 12, 14, 16, Rock salt.

Fig. 4.—Inclined beds, holding fossil plants. Carboniferous. South Joggins, Nova Scotia.

In [Fig. 4] we have a bed of coal and its accompaniments. The coal itself was produced by the slow accumulation of vegetable matter on a water-soaked soil, and this was buried under successive beds of sand and clay, now hardened into sandstone and shale, some of the beds holding trees and reed-like plants, which still stand on the soils on which they grew, and which must have been buried in sediment deposited in inundations or after subsidence of the land. In this section we may also observe that the beds are somewhat inclined; and that this is not their original position is shown by the posture of the stems of trees, once erect, but now inclined with the beds. This leads to a consideration very important with reference to our present subject; namely, that as our continents are mostly made up of beds deposited under water and afterwards elevated, these beds have in this process experienced such disturbances that they rarely retain their horizontal position, but are tilted at various angles. When we follow such inclined strata over large areas, we find that they undulate in great waves or folds, forming what are called anticlinal and synclinal lines, and that the irregularities of the surface of the land depend to a great extent on these undulations, along with the projection of hard beds whose edges protrude at the surface. In point of fact, as shown in [Fig. 5], mountain ranges depend on these crumplings of the earth’s crust; and the primary cause of these is probably the shrinkage of the mass of the earth owing to contraction in cooling. When the disturbances of beds are extreme, they often cause intricacies of structure difficult to unravel; but when of moderate extent they very much aid us in penetrating below the surface, for we can often see a great thickness of beds rising one from beneath another, and can thus know by mere superficial examination the structure of the earth to a great depth. It thus happens that geologists reckon the thickness of the stratified deposits of the crust of the earth at more than 70,000 feet, though they cannot penetrate it perpendicularly to more than a fraction of that depth. The two sections, [Figs. 6 and 7], showing the sequence of beds in England and in the northern part of North America, will serve, if studied by the reader, to show how, by merely travelling over the surface and measuring the upturned edges of beds, many thousands of feet of deposits may be observed, and their relative ages distinctly ascertained.

Fig. 5.—Ideal section of the Apalachian Mountains showing folding of the earth’s crust.

a, Anticlinal axes. b, Overturned strata. c, Synclinals. d, Unconformable beds.

In studying any extensive section of rock we find that its members may more or less readily be separated into distinct groups. Sometimes these are distinguished by what is termed unconformability, that is, the lower series has been disturbed or inclined before the upper has been deposited upon it. This is seen on a grand scale in the section [Fig. 7], in the case of the Laurentian and Cambrian formations, and on a smaller scale in [Fig. 8] in the unconformable superposition of Devonian conglomerate on Silurian slates at St. Abb’s Head. In the last section it is quite evident that the beds of the lower series have been bent into abrupt folds and worn away to a considerable extent before the deposition of the overlying series. In such a case we know not merely that the upper series is newer than the lower, but that some considerable time must have elapsed after the deposition of the one before the other was laid down; and we are not surprised to find that the fossils in the groups thus unconformable to each other are very different.

But even when the beds are conformable, they can usually be separated into groups, depending upon differences of mineral character, or changes which have occurred in the mode of deposition. One group of beds, for example, may be largely composed of limestone, another of sandstone or shale. One group may be distinguished by containing some special mineral, as, for example, rock salt or coal, while others may be destitute of such special minerals. One group may show by its fossils that it was deposited in the sea, others may be estuarine or lacustrine. Thus we obtain the means of dividing the rocks of the earth into groups of different ages, known as “Formations,” and marking particular periods of geological time. By tracing these formations from one district or region to another, we learn the further truth that the succession is not merely local, but that, though liable to variation in detail, its larger subdivisions hold so extensively that they may be regarded as world-wide in their distribution.

Fig. 6. Generalised section across England from Menai Straits to the Valley of the Thames.—After Ramsay.

0 Huronian? or Laurentian? 1 Cambrian and Lower Silurian. 2 Upper Silurian. 3 Devonian. 6, 7, 8 Trias and lias. 9 and 10 Jurassic. 11 Cretaceous. 12 Eocene.

Fig. 7.—Generalised section from the Laurentian of Canada to the coal-field of Michigan.

0 Laurentian (the Huronian is absent in the line of this section). 1 Cambrian. 2 Lower Silurian. 3 Upper Silurian. 4 Devonian. 5 Carboniferous.

Fig. 8.—Unconformable superposition of Devonian conglomerate on Silurian slates, at St. Abb’s Head, Berwickshire.—After Lyell.

Putting together the facts thus obtained, we can frame a tabular arrangement of the earth’s strata, as in the table prefixed to this chapter; and when we add the further discovery, very early made by geologists, that the successive formations differ from each other in their fossil remains, we have the means of recognising any particular formation by its fossils, even when the stratigraphical evidence may be obscure or wanting. Thus our knowledge of Epochs of Life, and indeed of the whole geological history of the earth, is based on the superposition of beds in the earth’s crust, and on the diversity of fossil remains in the successive beds so superimposed on each other; and it is on these grounds that we are enabled to construct a Table of Geological Formations representing the whole series of beds as far as known, with the characteristic groups of fossils of each period. Here I might close these preliminary considerations, but there are a few accessory questions, important to our clear comprehension of the subject, which may profitably occupy our attention for a short time.

One of these relates to the absolute duration of the time represented by the geological history of the earth. Such estimates as our present knowledge enables us to form are very indefinite. Whether we seek for astronomical or geological data, we find great uncertainty. To such an extent is this the case, that current estimates of the time necessary to bring the earth from a state of primitive incandescence to its present condition have varied from fifteen millions of years to five hundred millions. Of the various modes proposed, perhaps the most satisfactory as well as instructive is that based on the rate of denudation of our present continents, as indicated by the amount of sediment carried down by great rivers. The Mississippi, draining a vast and varied area in temperate latitudes, is washing away the American land at the rate of one foot in 6,000 years. The Ganges, in a tropical climate and draining many mountain valleys, works at the rate of one foot in 2,358 years. The mean of these two great rivers would give one foot in 4,179 years, at which rate our continents would be levelled with the waters in about six millions of years. But the land has been in process of renewal as well as of waste in geological time; and a better measure will be afforded by the amount of beds actually deposited. The entire thickness of all the stratified rocks of Great Britain has been calculated by Ramsay at 72,000 feet. Now, if we suppose the waste in all geological time to have been on the average the same as at present, and that this material has been deposited to the thickness of 72,000 feet on a belt of sea margin 100 miles in width, we shall have about 86 millions of years as the time required.[1] This has the merit of approximating to Sir William Thomson’s calculation, based on the rate of cooling of the earth, that a minimum of 100 millions of years may represent the time since a solid crust first began to form. As it is more likely that the rate of denudation has on the average been greater in former geological periods than at present, we may perhaps estimate fifty or sixty millions of years as the time required for the accumulation of all our formations. Some geologists object to this as too little, but in this some of them are influenced by the exigencies of theories of evolution, and others appear to have no adequate conception of the vast lapse of time represented by such numbers, in its relation to the actual rates of denudation and deposition.

It should be mentioned here, however, that, on certain theories now somewhat generally accepted, respecting the nature and source of solar heat, the absolute duration of geological time would be much reduced below the estimate of Sir Wm. Thomson. Prof. Tait has based on such data an estimate of fifteen millions of years. Prof. Simon Newcomb says that “on the only hypothesis science will now allow us to make respecting the source of the solar heat” (the gravitation hypothesis of Helmholtz) “the earth was, twenty millions of years ago, enveloped in the fiery atmosphere of the sun.” Dr. Kirkwood has called attention to these results in connection with the planetary hypothesis of La Place, in the Proceedings of the American Philosophical Society.[2] Should such views prove to be well-founded, geological calculations as to the time required for the successive formations may have to be revised.

If now we attempt to divide this time among the formations known to us, according to their relative thicknesses, we have, according to an elaborate estimate of Professor Dana, the time ratios of 12, 3, and 1 for the Palæozoic, Mesozoic, and Cainozoic periods respectively. Taking the whole time since the beginning of the Cambrian as forty-eight millions of years, we should thus have for the Palæozoic thirty-six millions, for the Mesozoic nine, and for the Tertiary three. Another calculation, recently made by Professors Hull and Haughton, gives the following ratios:—

This calculation is, however, based on the absolute thickness of the several series as ascertained in Great Britain, without reference to the nature of the beds, as indicating different rates of accumulation. Under either estimate it will be seen that the Palæozoic time greatly exceeds the Mesozoic and Cainozoic together, and consequently that changes of life seem to have proceeded at an accelerated rate as time wore on.

Another inquiry of some importance relates to the manner of preservation of fossils, and the extent to which they constitute the material of rocks. This inquiry is doubly important, as it bears on the genuineness of fossil remains, and on the means we have of understanding their nature.

Some rocks are entirely made up of matter that once was alive, or formed part of living organisms. This is the case with some limestones, which consist of microscopic shells, or of larger shells, corals, and similar calcareous organisms, either entire or broken into fragments and cemented together with pasty or crystalline limestone filling their interstices. This may be seen in [Fig. 9], which represents a magnified slice of a Silurian limestone. Coal in like manner consists of carbonised vegetable matter, retaining more or less perfectly its organic structure, and sometimes even the external forms of its constituent parts. More frequently, fossils are dispersed more or less sparsely through the substance of beds composed of earthy matter; and they have usually been more or less affected by chemical changes, or by mechanical pressure, or are mineralised by different substances which have either filled their pores by infiltration or have more or less completely replaced their substance. Of course, as a rule, the softer and more putrescible organic matters have perished by decay, and it is only the harder and more resisting parts that remain. Even these have often yielded to the enormous pressure to which they have been subjected, and if at all porous, have been changed by the slow action of percolating water charged with various kinds of mineral matter in solution.

Fig. 9.—Section of Trenton limestone, magnified, showing that it is composed of fragments of corals, crinoids, and shells. Montreal.

Fig. 10.—Diagram showing different state of fossilisation of a cell of a tabulate coral (Dawson’s Dawn of Life).

a Natural condition, wall calcite cell empty. b Wall calcite, cells filled with the same. c Walls calcite, cells filled with silica or a silicate. d Wall silicified, cells filled with calcite. e Wall silicified, cell filled with silica.

It thus happens that many fossils are infiltrated with mineral matter. Wood, for example, may have the cavities of its cells and vessels filled with silica or silicates, with sulphide or carbonate of iron, or with limestone, while the woody walls of the cells may remain either as coaly matter or charcoal. I have often seen the microscopic cells of fossil wood not only filled in this way, but presenting under a high power successive coats of deposit, like the banded structure of an agate.

In some cases not only are the pores filled with mineral matter, but the solid parts themselves have been replaced, and the whole mass has actually become stone, while still retaining its original structure. Thus silicified wood is often as hard and solid as agate, and under the microscope we see that the wood has entirely perished, and is represented by silica or flint, differing merely in colour from that which fills the cavities. In this case we may imagine the wood to have been acted on by water holding in solution silica, combined with soda or potash, in the manner of what is termed soluble glass. The wood, in decay, would be converted into carbon dioxide, and this as formed would seize on the potash or soda, leaving the silica in an insoluble state, to be deposited instead of the carbon. Thus each particle of the carbon of the wood, as removed by decay, would be replaced by a particle of silica, till the whole became stone. By similar chemical changes corals and shells are often represented by silica, or by pyrite, which has taken the place of the original calcareous matter; and still more remarkable changes sometimes occur, as when the siliceous spicules of sponges have been replaced by carbonate of lime. The organic matter present in the fossils greatly promotes these changes, by the substances produced in its decay, and thus it often happens that the shells, corals, etc., contained in limestone have been replaced by flint, while the inclosing limestone is unchanged. [Fig. 10] shows the various conditions which a coral may assume under these different modes of treatment.

The substance of a fossil may be entirely removed by decay

Fig. 11.—Cast of erect tree (Sigillaria) in sandstone, standing on a small bed of coal, South Joggins, Nova Scotia (Dawson’s Acadian Geology). or solution, leaving a mere mould representing its external form, and this may subsequently be filled with mineral matter, so as to produce a natural cast of the object. This is very common in the case of fossil plants; and large trunks of trees may sometimes be found represented, as seen in [Fig. 11], by stony pillars retaining nothing of the original wood except perhaps a portion of the bark in the state of coal. It sometimes happens that the substance of fossils has been removed, leaving mere empty cavities, sometimes containing stony cores representing the internal chambers of the fossils. Again, calcareous fossils imbedded in hard rocks are often removed by weathering, leaving very perfect impressions of their forms. For this reason the fossil remains contained in some hard resisting rocks can be best seen as impressed moulds on the weathered surfaces.

Fig. 12.—Protichnites septem-notatus. A supposed series of crustacean foot-prints made in sand, now hardened into sandstone. Cambrian.—After Logan.

Lastly, we sometimes have impressions or footprints representing the locomotion of fossil animals, rather than the fossils themselves. In this way some extinct creatures are known to us only by their footsteps on sand or clay, once soft, but now hardened into stone; and in the case of some of the lower animals the trails thus made are often not easily interpreted ([Figs. 12, 12a]). It has been found that even sea-weeds drifted by the tide make impressions of this kind, which, when they occur in old rocks, are very mysterious. Even rain-drops are capable of being permanently impressed on rocks, and constitute a kind of fossils. Besides these we have many kinds of imitative markings which simulate fossils, as those of concretions or nodules, which are often very fantastic in shape, those of dendritic crystallisation giving moss-like forms, and the complicated tracery produced on muddy shores by the little rills of water which follow the receding tide ([Fig. 13]). Such things are often mistaken by the ignorant for fossil remains, but are easily distinguished by a practised eye.

Fig. 12a.—Footprints of modern Limulus, or king-crab, in the sand, which enable us to interpret those in [Fig. 12].

The reader who has followed these, perhaps somewhat dry, details, will be rewarded for his patience by having some conception of the conditions in which we find fossil remains, and of the evidence by which we can refer these to different periods in the history of the earth.

Fig. 13.—Current markings on shale, resembling a fossil plant. Reduced from a photograph (Dawson’s Acadian Geology).

Carrying this knowledge with us, and at the same time glancing at the table of successive formations prefixed to this chapter, we shall be prepared, without any additional geological study, to understand the statements to be made in the following chapters, and to appreciate the actual nature of the succession of life in so far as it is at present known.

Magnified and Restored Section of a Portion of Eozoon Canadense.

The shaded portions show the animal matter of the Chambers, Tubuli, Canals, and Pseudopodia; the unshaded portions the calcareous skeleton.

CHAPTER II.

the beginning of life on the earth.

he day must have been when the first living being appeared for the first time on our planet. Was it plant or animal? or a generalised organism uniting in some mysterious way the properties and powers of two kingdoms of nature, now so distinct, and even contrary to each other in their manifestations? Did it appear suddenly, or was it slowly evolved from dead matter by some process in which the albuminous or protoplasmic matter, which we know forms the basal substance of living beings, was first produced and then endowed with life? Did the first living being appear in a mature state, or was it merely a germ from which the mature individual could be produced? These are questions which science in its present state has no means of answering. We do not know any process by which the ingredients of protoplasm can be combined so as to produce that substance without a previous living being. We do not know what molecular differences may exist between dead albumen and that which we see growing and moving and instinct with life; still less do we know how to set up or establish these differences. We do not know the precise nature or relation to other forces of the energy which actuates living organisms. In our experience the simplest creatures that have life spring from previous germs, themselves the products of previous generations of living beings. Thus we are in the presence of great mysteries which it might be impossible for us to solve, even if we were permitted to visit some new planet on which the dawn of life was breaking.

Some things, however, we can infer as to the conditions of the introduction of life.

First, there is every reason to believe that the earth we inhabit was once a glowing, incandescent mass, condensing from a vaporous condition, and quite unfit for the abode of living beings, and which, even if in some previous state its materials had constituted the mass of an inhabited world, must have lost every trace of any living germ in the fervent heat to which it had been subjected. There must, therefore, have been in some way an absolute creation or origination of life and organisation.

Secondly, we may infer that in the earlier stages of the earth, when it was perhaps wholly or almost entirely covered with the waters, when it was still uniformly warmed with its own internal heat, when it was surrounded with a pall of dense vapours preventing radiation, and nursing its heat within itself, though in a condition entirely unsuited to the higher forms of life, it may have presented circumstances more favourable to the origination and multiplication of living beings of low organisation than at any subsequent time. This incubation of creative power in the vaporous mantle over the primæval ocean was a favourite imagination of old thinkers, and is not obscurely hinted at in the Book of Genesis. It has been revived and much insisted on by evolutionists in our own time, though it has no certain foundation in scientific observation or experiment.

Thirdly, from the fact that plant-life alone has the power of subsisting on inorganic matter, and that plants furnish all the nourishment of animals, we may fairly infer that the life of the plant preceded that of the animal. It has, indeed, been suggested that some of the humbler forms of life may combine in a rude and simple way enough of the powers of the plant and the animal to enable them to bridge over the double gap between the animal and the plant, and the animal and the mineral, or that such creatures may in their early stages carry on vegetable functions, and in their later those of the animal. It is theoretically possible that life may have begun with such creatures, which some of the results of microscopical research would lead us to believe still exist. It is, however, on the whole more probable that simple plants first existed, and furnished pabulum to animals of low grade introduced almost contemporaneously.

Fourthly, all our knowledge of the succession of life leads us to believe that it was not the higher plants and animals that first sprang into existence from the teeming earth, but creatures of low and humble organisation, suited to the then immature and unfinished condition of the planet. It is also in accordance with the amazing fecundity of the seas in all geological periods in these lower forms of life, to suppose that the earliest living things originated in the waters, and that the plants and animals of the land are of later date.

Do we know anything from actual observation of this earliest population of the world? Such knowledge we can hope to acquire only by studying the oldest formations known to us; and these, it must be confessed, exist in a state so highly crystalline, and so much affected by internal heat, by mechanical pressure, and by movement, as to render it little likely that organic remains should be preserved in them in a state fit for recognition.

In many parts of the world, and notably in Canada and Scandinavia, as well as in Wales, Scotland, and Bavaria, the older Palæozoic rocks, the lowest containing plants in great abundance, rest on still older crystalline beds, which have become hard and crystalline in pre-Palæozoic times, and have contributed sand and pebbles to the succeeding very ancient deposits. These old rocks—the Eozoic series of our table—may be grouped in two great systems, the Laurentian and Huronian ([Fig. 14]). The former may be conveniently divided into three members: First, the Bojian, or Ottawa gneiss, consisting of stratified granite rocks, usually of a red colour, and of very great thickness. This contains, so far as known, no limestone, and has afforded as yet no trace of fossils. Secondly, the Middle Laurentian, the greater part of which consists of gneiss, but containing important beds of other rocks, as quartzite, iron ore, and limestone. It is in this series that we have the first evidence of life, and it is here also that we find the greatest abundance of carbon, in the form of graphite or plumbago, and also large quantities of calcium phosphate, or bone earth. Thirdly, the Upper Laurentian or Norian series. This consists in great part of Labadorite, or lime feldspar, but has also beds of ordinary gneiss, limestone, and iron ore.

Fig. 14.—Ideal section, showing the relations of the Laurentian and Huronian.

a, Lower Laurentian. b, Middle Laurentian. c, Upper Laurentian. d, Huronian. e, Cambrian and Silurian.

The latter, the Huronian, is much less crystalline, and is divisible into two series—the Lower Huronian, which includes many beds of volcanic origin, and the Upper Huronian, which has afforded some obscure fossils. The Huronian was first recognised by Sir W. E. Logan in Canada, but corresponding rocks exist in Europe. The Pebidian series of Hicks in Wales is probably of this age.

It is likely that much of the present appearance and condition of the most ancient rocks may be attributed to metamorphism, that is, to the slow baking under the influence of heat, heated water, and pressure, to which they have been subjected in the lower parts of the earth’s crust, when buried deeply under newer deposits. It is also true, however, as Dr. Sterry Hunt has pointed out in detail, that they present mineral characters which show a mode of deposition different from that which has prevailed subsequently, and probably indicating great ejections of heated mineral matter into the primitive ocean, and comparatively little of that deposit therein of mere sand and clay which has prevailed in subsequent geological periods. In short, these rocks have an unmistakably primitive aspect, distinguishing them from those of later times, and conveying the impression that they approach at least to the records of that time when a heated ocean first rested on the thin and recently solidified crust of our planet. If this is really the case, then our Lower Laurentian—hard, compact, destitute of limestone, and composed of material which may be little else than the débris of products of internal heat merely spread out into bedded forms by water—may represent a time when no living thing as yet tenanted the waters; and the dawn of life may have appeared in that period when the Middle Laurentian beds were laid down. Here at least we find two kinds of evidence pointing to the existence of certain forms of life in the waters.

The first depends on the mineral character of the beds themselves. This formation holds several very thick beds of limestone. Now although this kind of rock may, under certain circumstances, be deposited directly from solution in water, it is not ordinarily so deposited, but more usually through the agency of living beings inhabiting the waters, and forming their skeletons or hard parts of limestone derived from the water, usually through the medium of humble forms of plant life. In this way are formed reefs of coral and beds of shells and of chalky ooze, all composed of material once constituting the skeletons of animals. The study of limestones of all geological ages shows that this has been the usual mode of their formation. If the Laurentian limestones had a similar origin, the seas of that period must have swarmed with animals having calcareous coverings; and the study of more modern limestones which have become highly crystalline shows that it is quite possible that the forms and structures of these organisms may have been obliterated.

Again, the Middle Laurentian abounds in carbon or coaly matter. True, this is in the form of graphite or plumbago, but this condition may be a result of metamorphism; and we know that the carbon of coal-beds and bituminous shales of much more modern times has been altered into graphite. Further, the graphite occurs in the way in which we should expect it to occur if of organic origin. It is found disseminated in the limestone, just as bituminous matter is found in unaltered rocks of this kind. It is found interlaminated with gneiss, as carbonaceous and bituminous matters are found in the shales of the ordinary fossiliferous rocks, where these substances are known to be of organic origin. The graphite also occurs in a very pure form in irregular veins, just as in some bituminous formations the rock oil, oozing into fissures, has been hardened into asphalt or coaly matter.[3]

To these facts may be added the presence of thick beds and veins of iron ore and of apatite or calcium phosphate (bone earth). Both of these substances occur in a disseminated state in nearly all rocks, but they are concentrated into definite deposits by the action of life. Iron is usually dissolved out and redeposited by acids produced in the decay of vegetable matter, as we see in the clay ironstones of the coal formation and in bog-iron ores. Calcic phosphate is taken up by many animals, and forms their shells or skeletons, and on their death is deposited in beds on the sea-bottom, sometimes to a very considerable extent.

The concurrence of all these phenomena in the Middle Laurentian may be held to afford a strong presumption that, could we discover these rocks in an unaltered state, we should find the limestones filled with marine fossils and the graphite showing the forms or structure of plants. The only startling feature in this conclusion is, that if we admit it, we must also admit that life was developed in the Laurentian time in an exuberance not surpassed, if equalled, in any subsequent period. Still, there is nothing incredible in this, for if the forms of life were few and low, their increase may have been rapid, because unchecked; and they no doubt found in the ancient seas a surplusage of material on which to feed and with which to construct their skeletons. Dr. Hunt has estimated that the amount of carbon now sealed up as coaly matter would, if diffused in the atmosphere as carbon dioxide, afford 600 times the quantity of that gas at present floating in the air. A still more vast amount is sealed up in the limestone of the several geological formations. The same chemist has shown that the quantity of lime held in solution in the ocean must have been much greater in Laurentian times than at present. These facts at least allow us to suppose that in the Eozoic times there were great supplies of carbon and of lime available to such creatures of low organisation as were capable of profiting by them; and we have no reason to doubt that there may have been plants and animals so constituted as to flourish in conditions of this kind, in which perhaps scarcely any modern species could exist.

These probabilities have caused geologists anxiously to search for any traces of fossil organic remains in the old Laurentian rocks; and they have been rewarded by the discovery of one species, Eozoon Canadense, still often referred to as only a problematical fossil; but this arises to a large extent from the prevalent want of knowledge sufficient to appreciate the evidence for its organic character. This being once admitted, we have in the existence of Eozoon alone a sufficient cause for the accumulation of much of the Laurentian limestone, though there is reason to believe that it was not the only inhabitant of those ancient seas.

Fig. 15 (Nos. 1 to 4).—Small weathered specimen of Eozoon. From Petite Nation.

1, Natural size; showing general form, and acervuline portion above and laminated portion below. 2, Enlarged casts of cells from upper part. 3, Enlarged casts of cells from the lower part of the acervuline portion. 4, Enlarged casts of sarcode layers from the laminated part.

The best specimens of Eozoon occur as rounded, flattened, or more or less irregular lumps or masses in certain layers of the Laurentian limestone. When weathered on the surface of the rock, these lumps show a regular concentric lamination, caused by thin fibres of limestone, alternating with other mineral substances, filling up the spaces between them. When these intervening layers are composed of such minerals as Serpentine, Loganite, Pyroxene, or Dolomite, which are more resisting than the limestone, they project when weathered, or when the limestone is etched by an acid, so as to show the lamination very distinctly. At the lower surface of the masses the layers are seen to be thicker than they are above, and in perfect specimens they are seen toward the surface to break up into small rounded vesicles of calcite, like little bubbles, which constitute the so-called acervuline condition of Eozoon ([Fig. 15], No. 2). Slices of the fossil etched with an acid show these appearances very perfectly, and can even be printed from, so as to present perfect nature-prints of the structure ([Fig. 16]).

Fig. 16.—Nature-printed specimen of Eozoon slightly etched with acid. It shows the lamination, and at one side fragmental Eozoon (Life’s Dawn on Earth).

On etching a small fragment or slice with very dilute acid, so as to dissolve away the calcite slowly, if the specimen be well preserved, we find that the calcite layers have a very curious structure. This is indicated by the appearance of little white or transparent threads of Serpentine, Dolomite, or Pyroxene, which ramify throughout the substance of the limestone layers, and are left intact when they have been dissolved. These little processes must originally have been pores in the limestone layers, which have been filled with the substance which constitutes the alternate laminæ. In addition to this, if we use a somewhat high microscopic power, and especially if we study the structures as seen in thin transparent slices, we can perceive a still finer tubulation along the sides of the calcite layers, represented by extremely minute parallel rods of mineral matter ([Figs. 17, 18]).

Now if we regard these structures as those of an infiltrated fossil, as described in last chapter, their interpretation will not be difficult. The original organism was composed of calcareous matter in thin concentric laminæ, connected with each other by pillars and plates of similar material. Between these laminæ was lodged the soft, jelly-like substance of a marine animal, growing by the addition of successive layers, each protected by a thin calcareous crust. The layers were originally traversed by very numerous parallel tubuli, permitting the soft protoplasm to penetrate them; and when, in the progress of growth, it was necessary to strengthen these layers, they were thickened by a supplemental deposit traversed by larger and ramifying canals. When the animal was dead, and its soft parts removed by decay, the chambers between the laminæ, as well as the minute canals and tubuli, became infiltrated with mineral matter, in the manner described in the last chapter, and when so preserved became absolutely imperishable under any circumstances short of absolute fusion.

Fig. 17.—Magnified group of canals in supplemental skeleton of Eozoon.

Taken from the specimen in which they were first recognised (Life’s Dawn on Earth).

Fig. 18.—Portion of Eozoon magnified 100 diameters, showing the original cell-wall with tubulation, and the supplemental skeleton with canals.—After Carpenter.

a, Original tubulated wall or “Nummuline layer.” More magnified in Fig. A. b, c, Intermediate skeleton, with canals.

This interpretation leads to the conclusion, at which I arrived from the study of the first well-preserved specimen ever submitted to microscopic examination, that the animal which produced the calcareous skeleton of Eozoon was a member of that lowest grade of Protozoa known as Foraminifera; and which, after living through the whole of geological time, still abound in the sea. The main differences are, that Eozoon presents a somewhat generalised structure, intermediate between two modern types, and that it attained to a gigantic size compared with most of these organisms in later periods. How near it approaches in structure to some modern forms may be seen by comparison of the recent species represented in [Fig. 19], in which the parts corresponding to the chambers, laminæ, tubuli, and canals of Eozoon can be readily distinguished.

Fig. 19.—Magnified portion of shell of Calcarina.—After Carpenter.

a, Cells. b, Original cell-wall with tubuli. c, Supplementary skeleton with canals.

The modern animals of this group are wholly composed of soft gelatinous protoplasm or sarcode, the outer layer of which is usually somewhat denser than the inner portion; but both are structureless, except that the inner layer may present a more or less distinct granular appearance. Many of them show a distinct spot or cell, called the nucleus, and some have minute transparent vesicles, which contract and expand alternately, and appear to be of the nature of circulatory or excretory organs. They have no proper alimentary canal, but receive their food into the general mass and digest it in temporary cavities. Their means of locomotion and prehension are soft thread-like or finger-like processes, extended at will from the surface of any part of the body, and known as false feet (pseudopodia). From these processes the whole group has obtained the name of Rhizopods, or rootfooted animals. They may be regarded as constituting the simplest and humblest form of animal life certainly known to us.

The very numerous species of these creatures existing in the waters of the modern world may be arranged under three principal groups. The first and highest includes those which have lobate or finger-like pseudopods, and a well-developed nucleus and pulsating vesicle ([Fig. 20], a). They are mostly inhabitants of fresh water, and destitute of a hard crust or shell. A second group, including many inhabitants of the sea as well as of fresh waters, has thread-like radiating pseudopodia[4] ([Fig. 20] b). Some of these form beautiful silicious skeletons. A third group, essentially marine, consists of those with reticulated pseudopodia, and usually destitute of distinct nucleus and pulsating vesicle ([Fig. 21]). They produce beautiful calcareous skeletons, often very complex, or sometimes are content to cover themselves with a crust of agglutinated grains of sand. It is to this last group that Eozoon belongs, and to the highest division of it—that which has the shell perforated with minute pores, often of two kinds. It is curious that just as we have the chambers and pores of Eozoon filled with serpentine, so in all geological formations and in the modern seas it is not uncommon to find Foraminifera having their cavities filled with glauconite and other hydrous silicates allied to serpentine.

Fig. 20.—a, Amœba, a fresh-water naked Rhizopod; and b, Actinophrys, a fresh-water Protozoon of the group Radiolaria, with thread-like pseudopodia.

Fig. 21.—Nonionina, a modern marine Foraminifer. Showing its chambered shell and netted pseudopodia.—After Carpenter.

If we attempt to trace the Rhizopods onward from the Middle Laurentian, we are met with a great hiatus in the Upper Laurentian. The species Eozoon Bavaricum has, however, been found in rocks apparently of Huronian age; but this is the last known appearance of Eozoon, properly so-called. In the Cambrian or Siluro-Cambrian, however, we meet with many gigantic Protozoa, more especially those known as Stromatopora, Archæocyathus, Receptaculites, and Cryptozoon.

Fig. 22.—Stromatopora concentrica.—After Hall.

a, Section of the same, magnified. b, Small portion highly magnified, showing laminæ and pillars.

The typical Stromatoporæ, or Layer-corals, consist, like Eozoon, of concentric layers, connected by numerous pillars, which are often, though not always, more definite and regular than in the Laurentian fossil. The laminæ are perforated, but more coarsely than in Eozoon, and they are often thickened with supplemental deposit which, in some of the forms, presents canals radiating from vertical tubes or bundles of tubes penetrating the mass ([Figs. 22, 23]). The mode of growth of Stromatopora must have closely resembled that of Eozoon, and the forms produced are so similar that it is often quite impossible to distinguish them by the naked eye. Like Eozoon, they form the substance of important limestones, and single masses are sometimes found as much as three feet in diameter. The Stromatoporæ extend from the Upper Cambrian to the Devonian inclusive. In the Carboniferous they are continued by smaller and more regular organisms of the genus Loftusia,[5] and this genus seems to extend without marked change up to the Eocene Tertiary. Recent students of the Stromatoporæ seem disposed to promote them from the province of Protozoa to that of the Hydroids.[6] The reasons for this seem cogent in the case of some of the forms, but in my judgment fail in others, more especially in the older forms. It may ultimately be found that the group as now held includes very different types of structure. In modern times I know of no nearer representative than the animal whose skeleton often adheres in red encrusting patches to our specimens of corals, and which is known as Polytrema. In general structure it is not very far from being a very degenerate kind of Stromatopora.

Fig. 23.—Caunopora planulata. Showing the radiating canals on a weathered surface. Devonian.—After Hall.

It is curious that in the line of succession above stated, the beautiful tubulated cell-wall of Eozoon disappears; and this structure seems, after the Laurentian, to be for ever divorced from the great laminated Protozoans. It reappears in the Carboniferous, in certain smaller organisms of the type of the Nummulites, or Money-stone Foraminifers, and is continued in this group of smaller and free animals down to the present time. In the Cretaceous and early Tertiary periods, the Foraminifera of different types have been nearly as great rock-builders as they were in the Laurentian. Some of these later rock-builders, however, have belonged to the lower or imperforate group; others to the higher or Rotaline and Nummuline groups; and, as a whole, they have been individually small, making up in numbers what they lacked in size. Probably the conditions for enabling animals of this type rapidly, and on a large scale, to collect calcareous matter, were more favourable in the Laurentian than they have ever been since.

Fig. 24.—Archæocyathus minganensis. A Primordial Protozoon.—After Billings.

a, Pores of the inner wall.

In the Siluro-Cambrian age two other forms of gigantic Foraminiferal Protozoans were introduced, widely different from Eozoon, and destined apparently not to survive the period in which they appeared. These were Archæocyathus, the ancient Cup-corals, and Receptaculites, which may perhaps be called the Sack-corals. Both are quite remote from Eozoon in structure, wanting its complexity in the matter of minute tubules, and having greater regularity and complication on the large scale. Archæocyathus had the form of a hollow inverted cone with double perforated walls, connected by radiating irregular plates, also perforated ([Fig. 24]). It has been regarded as a sponge, and some species are certainly accompanied with spicules; but these I have ascertained to be merely accidental, and will be referred to in the next chapter. The true structure of Archæocyathus consists of radiating calcareous plates enclosing chambers connected by pores. Archæocyathus came in with the Later Cambrian, and seems to have died out in the Siluro-Cambrian. The only more modern things which at all resemble it are the Foraminifera called Dactylopora, which belong to the Tertiary period.

Fig. 25.—Receptaculites. Restored.—After Billings.

a, Aperture. b, Inner wall. c, Outer wall. n, Nucleus, or primary chamber. v, Internal cavity.

Receptaculites is a still more complex organism. It has a sack-like form, often attaining a large size, and the double walls are composed of square or rhombic plates, connected with each other by hollow tubes from which proceed canals perforating the plates ([Fig. 25]). This curious structure is confined to the Siluro-Cambrian, and is so dissimilar from modern forms that its affinities have been subject to grave doubts.

Fig. 26.—Section of Loftusia Persica. An Eocene Foraminifer. Magnified five diameters.—After Carpenter and Brady.

We thus have presented to us the remarkable fact that in the Palæozoic age we have no precise representative of Eozoon, but instead three divergent types, differing from it and from each other, all apparently specialised to particular uses, all temporary in their duration; while in later times nature seems to have returned nearer to the type of Eozoon, though on a smaller scale, and separating some characters conjoined in it. Some portion of this curious result may be due to our ignorance; and it would be interesting to know, what we may know some day, how this type of life was represented in the long interval between the Huronian and the Upper Cambrian, when perhaps there may have been forms that would at least enable us to connect Eozoon and Stromatopora.

Another link in the chain of being remains to be noticed here. In the Laurentian limestones we meet with numerous minute spherical bodies and groups of spheres with calcareous tubulated tests.[7] These may either be small Foraminiferæ, distinct from Eozoon, or may be germs or detached cells from its surface. Similar bodies are found in the lower part of the Siluro-Cambrian, in the Quebec group at Point Levis; and there they are filled with a species of glauconite constituting a sort of greensand rock. Still higher, in the Carboniferous, there are very numerous species of Foraminifera, presenting forms very similar to those in the modern seas, so that in the smaller shells of this group we seem to have evidence of a continuous series all the way from the Laurentian to the present time. The greater laminated forms co-exist with these up to the Eocene Tertiary. Throughout the whole of geological time—from the formation of the Laurentian limestones to that of the chalky ooze accumulating in the modern ocean—these humble creatures have been among the chief instruments in seizing on the calcareous matter of the waters and depositing it in the form of limestone.

Fig. 27.—Foraminiferal Rock Builders, in the Cretaceous and Eocene.

a, Nummulites lævigata—Eocene. b, The same, showing chambered interior. c, Milioline limestone, magnified—Eocene, Paris. d, Hard Chalk, section magnified—Cretaceous.

I have said nothing of the development of higher forms of animal life from Eozoon, simply because I know nothing of it. We shall see in the next chapter that these are introduced seemingly in an independent manner. We may be content to trace foraminiferal life along its own line of development, waxing and waning, but ever confined within the same general boundaries, from the Laurentian to the present time. It is likely that if, in any of the ages constituting this vast lapse of time, a dredge had been dropped into the depths of ocean, it would have brought up Foraminifera not essentially different in form and structure. If any one asks to what extent the successive species constituting this almost endless chain may be descendants one of the other, we have no absolutely certain information to give. On the one hand, it is not inconceivable that such forms as Stromatopora or Nummulina may have descended from Eozoon. On the other hand, it is equally conceivable that the same power which produced Eozoon at first, whether from dead matter or from some unknown lower form of life, may have repeated the process in later times with modifications. In any case it is probable that the Foraminifera have experienced alternations of expansion and shrinkage, of elevation and decadence, in the lapse of geological time. There were times in which many new forms swarmed into existence, and times in which old forms were becoming extinct without being replaced by others. In so far as the areas of the continents and the adjacent waters are concerned, those periods when the land was subsiding under the ocean must have been their times of prosperity, those in which the crust of the earth shrunk and raised up large areas of land must have been their times of decay. Still this lowest form of animal life has never perished, but has always found abundant place for itself, however pressed by physical change and by the introduction of higher beings.

Paradoxides Regina (Matthews). Lower Cambrian of New Brunswick.
1/6th Nat. Size.

CHAPTER III.

the age of invertebrates of the sea.

f the middle portion of the Laurentian age was really a time of exuberant and abounding life, either this met with strange reverses in succeeding periods, or the conditions of preservation have been such as to prevent us from tracing its onward history. Certain it is, that according to present appearances we have a new beginning in the Cambrian, which introduces the great Palæozoic age, and few links of connection are known between this and the previous Eozoic.

At the beginning of the Palæozoic we have reason to believe that our continents were slowly subsiding under the sea, after a period of general continental elevation which was consequent on the crumbling of the earth’s crust at the close of the Eozoic; and on the new sea-bottoms formed by this subsidence came in, slowly at first, but in ever-increasing swarms, the abundant and varied life of the early Palæozoic.

In the oldest portion of the Cambrian series in Wales, Hicks has catalogued species of no less than seventeen genera, embracing Crustaceans, the representatives of our crabs and lobsters, bivalve and univalve shell-fishes of different types, worms, sea-stars, zoophytes, and sponges. If we could have walked on the shores of the old Cambrian sea, or cast our dredge or trawl into its depths, we should have found representatives of most of the humbler forms of sea life still extant, though of specific forms strange to us. Perhaps the nearest approach to such experience which we can make is to examine the group of Cambrian animals delineated in [Fig. 28], and to notice, under the guidance of the geologist above named, the sections seen at St. David’s, in South Wales.

Fig. 28.—Group of Cambrian Animals (from Nicholson).

a, Arenicolites didymus, worm tubes. b, Lingulella ferruginea. c, Theca Davidii. d, Modiolopsis solvensis. e, Orthis Hicksii. f, Obolella sagittalis. g, Hymenocaris vermicauda. h, Trilobite, Olenus micrurus.

Here we find a nucleus of ancient rocks supposed to be Laurentian, though in mineral character more nearly akin to the Huronian, but which have hitherto afforded no trace of fossils. Resting unconformably on these is a series of partially altered rocks, regarded as Lower Cambrian, and also destitute of organic remains. These have a thickness of almost 1,000 feet, and they are succeeded by 3,000 feet more of similar rocks, still classed as Lower Cambrian, but which have afforded fossils. The lowest bed which contains indications of life is a red shale, perhaps a deep-sea bed, and possibly itself partly of organic origin, by that strange process of decomposition or dissolution of foraminiferal ooze and volcanic fragments, going on in the depths of the modern ocean, and described by Dr. Wyville Thomson as occurring over large areas in the South Pacific. The species are two Lingulellæ, a Discina and a Leperditia. Supposing these to be all, it is remarkable that we have no Protozoa or Corals or Echinoderms, and that the types of Brachiopods and Crustaceans are of comparatively modern affinities. Passing upward through another 1,000 feet of barren sandstone, we reach a zone in which no less than five genera of Trilobites are found, along with Pteropods and a sponge. Thus it is that life comes in at the base of the Cambrian in Wales, and it may be regarded as a fair specimen of the facts as they appear in the earlier fossiliferous beds succeeding the Laurentian. Taking the first of these groups of fossils, we may recognise in the Leperditia a two-valved Crustacean closely allied to forms still living in the seas and fresh waters. The Lingulellæ, whether we regard them as molluscoids, or, with Professor Morse, as singularly specialised worms, represent a peculiar and distinct type, handed down, through all the vicissitudes of the geological ages, to the present day. The Pteropods and the sponge are very similar to forms now living. The Trilobites are an extinct group, but closely allied to some modern Crustaceans. Had the primordial life begun with species altogether inscrutable and unexampled in succeeding ages, this would no doubt have been mysterious; but next to this is the mystery of the oldest forms of life being also among the newest. Whatever the origin of these creatures, they represent families which have endured till now in the struggle for existence without either elevation or degradation. Yet, though thus vast in their duration, they seem to have swarmed in together and in great numbers, in the Cambrian, without any previous preparation. From the Cambrian onward, throughout the whole Palæozoic, there is no decided break in the continuity of marine life; and already in the Silurian period the sea was tenanted with all the forms of invertebrate life it yet presents, and these in a teeming abundance not surpassed in any succeeding age. Let us now, in accordance with our plan, select some of these ancient inhabitants of the waters and trace their subsequent history.

Remains of sea-weeds are undoubtedly present in the Cambrian rocks. One of the lowest beds in Sweden has been named from their abundance the Fucoidal Sandstone; and wherever fossiliferous Cambrian rocks occur, some traces, more or less obscure, of these plants may be found. Nearly all that we can say of them, however, is, that, in so far as their remains give any information, they are very like the plants of the same group that now abound in our seas. In the fucoidal sandstone of Sweden certain striated or ribbed bodies have been found, which have even been regarded as land plants;[8] but they seem rather to be trails or marks left by sea-weeds dragged by currents over a muddy bottom. The plants of the sea thus precede those of the land, and they begin on the same level as to structure that they have since maintained. I agree with Nathorst, however, in holding that the Bilobites and many other forms believed by some to be sea-weeds, are really trails and tracks of animals.[9]

The Foraminifera of the Palæozoic we have noticed in the last chapter; but we now find a new type of Protozoan—that of the Sponge. Sponges as they exist at present may be defined to be composite animals, made up of a great number of one-celled or gelatinous zoids, provided with vibrating threads or cilia, and so arranged that currents of water are driven through passages or canals in the mass, by the action of the cilia, bringing food and aerated water for respiration. To support these soft sarcodic sponge-masses, they secrete fibres of horny matter and needles (spiculæ) of flint or of limestone, forming complicated fibrous and spicular skeletons, often of great beauty. They abound in all seas, and some species are found in fresh waters.

Fig. 29.—Portion of skeleton of Hexactinellid Sponge (Cœloptychium). Magnified. After Zittel.

With the exception of a very few species destitute of skeleton, and which we cannot expect to find in a fossil state, the sponges may be roughly divided into three groups: 1, those with corneous or horny skeleton, like our common washing sponges; 2, those with skeletons composed of silicious needles of various forms and arrangement; 3, those with calcareous spicules. Of these, the second or silicious group has precedence in point of time, beginning in the Early Cambrian, and continuing to the present. Two of its subdivisions are especially interesting in their range. The first is that of the Lattice-sponges (Hexactinellidæ), in which the spicules have six rays placed at right angles, and are attached to each other by their points, so as to form a very regular network ([Fig. 29]). The second is that of the Stone-sponges (Lithistidæ), in which the spicules are four-rayed or irregular, and are united by the branching root-like ends of the rays. The most beautiful of all sponges, the Venus Flower-basket (Euplectella), is a modern Hexactinellid, and the wonderful weaving of its spicules is as marvellous a triumph of constructive skill as its general form is graceful. The Lithistids are less beautiful, but are the densest and most compact of sponges, and are represented by several species in the modern seas. Both of these types go back to the Early Cambrian, and have continued side by side to the present day, as representatives of two distinct geometrical methods for the construction of a spicular skeleton.

Fig. 30.—Protospongia fenestrata (Salter). Menevian group.

a, Fragment showing the spicules partially displaced. b, Portion enlarged.

Fig. 31.—Astylospongia præmorsa (Roemer). Niagara group.—After Hall.

a, Spicules magnified.

Fig. 32.—Spicules of Lithistid sponge (Trichospongia of Billings). From the Cambrian of Labrador.

Many years ago the keen eye of the late lamented Salter detected in a stain on the surface of a slab of Cambrian slate the remains of a sponge; and minute examination showed that its spicules crossed each other, and formed lattice-work on the hexactinellid plan. Salter boldly named it Protospongia (the first sponge), and it is still the earliest that we know (Fig. 30). Thus the type whose skeleton is the most perfect in a mechanical point of view takes the lead. It is continued in the Silurian in many curious forms, of which the stalkless sponges (Astylospongia) are the most common ([Fig. 31]). It perhaps attains its maximum in the Cretaceous, from which the beautiful example in [Fig. 29] is taken, and it still flourishes, giving us the most beautiful of all recent forms. Before the close of the Cambrian there were other sponges of the Lithistid type. [Fig. 32] represents a group of spicules from the Calciferous (Lowest Silurian or Upper Cambrian) of Mingan,[10] and which probably belong to a large Lithistid sponge of that early time. The Lithistids have been recognised in the Upper Silurian and Carboniferous, and continuing upward to the Cretaceous, there become vastly numerous, while their modern representatives are by no means few. The silicious sponges with simple spicules appear to have existed as far back as the Siluro-Cambrian, and there is believed to be almost as early evidence of horny or corneous sponges. The calcareous sponges have been recognised as far back as the Silurian.[11] Thus from the close of the Palæozoic all the types of sponges seem to have existed side by side; and in the Cretaceous period, when such large areas of our continents were deeply submerged, they attained a wonderful development, perhaps not equalled in any other era of the earth’s history.

Sponges may be regarded as the highest or most complex of the Protozoa or the lowest of the Coelenterates. We have no links wherewith to connect them with the lower Protozoa of the Eozoic period; and through their long history, though very numerous in genera and species, they show no closer relationship with the Foraminifera below, and the Corals above, than do their successors in the modern seas. They thus stand very much apart; and modern studies of their development and minute structures do not seem to remove them from this isolation. Though we are treating here of inhabitants of the sea, it may be proper to mention that Geinitz has described two species from the Permian which he believed to be early precursors of the Spongillæ, or fresh-water sponges; but more recently he seems to regard them as probably Algæ. Young has, however, recently found true spicules of Spongilla in the Purbeck beds.[12]

Fig. 35.—Dictyonema Websteri (Dn). Niagara formation.

a, Enlarged portion (Acadian Geology).

Fig. 36.—Group of modern Hydroids allied to Graptolites. Magnified, and natural size.

a, Sertularia. b, Tubularia. c, Campanularia.

Fig. 37.—Silurian Graptolitidæ.

a, Graptolithus. b, Diplograpsus. c, Phyllograpsus. d, Tetragrapsus. e, Didymograpsus.

A stage higher than the sponges are those little polyp-like animals with sac-like bodies and radiating arms or tentacles, which form minute horny or calcareous cells, and bud out into branching communities, looking to untrained eyes like delicate sea-weeds—the sea-firs and sea-mosses of our coasts ([Fig. 36]). These belong to a very old group, for in the oldest Cambrian we have a form referred to this type ([Fig. 33]), and in the Upper Cambrian another still more decided example ([Fig. 34]).[13] This style of life, once introduced, must have increased in variety and extended itself with amazing rapidity, for in the Siluro-Cambrian age we find it already as characteristic as in our modern seas, and so abundant that vast thicknesses of shale are filled and blackened with the débris of forms allied to the sea-firs, and masses of limestone largely made up of the more calcareous forms of the sea-mosses. As examples of the former we may take the Graptolites, so named from their resemblance to lines of writing, and of which several forms are represented in [Fig. 37]. The little teeth on the sides of these were cells, inhabited probably by polyps, like those represented in the modern Sertularia in [Fig. 36]. Some of them were probably attached to the bottom. In others the branches radiated from a central film which may have been a hollow vesicle or float, enabling them to live at the surface of the water ([Fig. 38]). These Graptolites are specially characteristic of the Upper Cambrian and Lower Silurian. The netted ones (Dictyonema), as may be seen from [Figs. 34 and 35], came in before the close of the Cambrian, and continue unchanged to the Silurian, where they disappear. The branching forms, seen in [Fig. 37], have scarcely so great a range. They thus form most certain marks of the period to which they belong, and being oceanic and probably floaters, they diffused themselves so rapidly that they appear to indicate the same geological time in countries so widely separated as Europe, North America, and Australia. It is curious, too, that while the Graptolites thus mark a definite geological time, and seem to disappear abruptly and without apparent cause, they are the first link in the long chain of the Hydroids, which, though under different family forms, continue to this day, apparently neither better nor worse than their perished Palæozoic relatives. There is a group of little Stony Corals (Monticuliporidæ), which were possibly also the cells of Hydroids, that have a similar history. They are the only known Corals that date so far back as the Upper Cambrian; and they continue under very similar forms all through the Palæozic, and are represented by the millepore corals of the present day. [Fig. 40] represents a form found at the base of the Siluro-Cambrian, and [Fig. 41] shows forms characteristic of the Carboniferous Limestone.

Fig. 39a.—Fenestella Lyelli (Dawson). A Carboniferous Bryozoan.

If we turn now to the sea-mosses (Bryozoa), we have a group of minute polyp-like animals

Fig. 40.—Chaetetes fibrosa. A tubulate coral with microscopic cells. Siluro-Cambrian. inhabiting cells not unlike those of the Hydroids, and which form plant-like aggregates. But the animals themselves are so different in structure that they are considered to be nearer allies of the bivalve shell-fishes than of the Corals. They are, in short, so different, that the most ardent evolutionist would scarcely hold a community of origin between them and such creatures as the Graptolites and Millepores, though an ordinary observer might readily confound the one with the other. These animals appear at the beginning of the Siluro-Cambrian, and such forms as that represented in [Fig. 39], very closely allied to some now living, are large constituents of some of the limestones of that period. Other forms, like that represented in [Fig. 39]a, are very characteristic of the Carboniferous. These animals, individually small, though complicated in structure and branching into communities, scarcely ever of any great magnitude, humble creatures which have never played any great part in the world, have, nevertheless, been so persistent that, though specific and generic forms have been changed, the group may be said to be in the modern seas exactly what it was in those of the early Palæozoic, nor can it be affirmed to have originated in anything different, or to have produced anything.

Fig. 41.—a, Stenopora exilis (Dawson). b, Chaetetes tumidus (Edwards and Haine). Carboniferous.

The true Stony Corals (Anthozoa) are as yet unknown in the Cambrian. They entered on the stage in immense abundance in the Siluro-Cambrian, where considerable limestones are largely composed of their remains, mixed, however, and sometimes overpowered with those of Bryozoa and Hydroids. An ordinary coral, such as those of which coral reefs are built—the red coral, used for ornament is not quite similar—is the skeleton of an animal constructed on the plan of a sea anemone; with a central stomach surrounded by radiating chambers, and having above a crown of tentacles. The stony coral surrounds and protects the soft body of the animal, and may either be a single cell, for one animal, or an aggregation of such cells, constituting a rounded or branching mass. The modern star coral, represented in [Fig. 42], is an instance of the latter condition. It shows nineteen or twenty animals, each with a central mouth and fringe of short tentacles, aggregated together, and two of them showing the spontaneous division by which the number of animals in the mass is progressively increased. The living coral shows only the soft animals and the animal matter connecting them; but if dead there would be a white stony mass with a star-like cell or depression corresponding to each animal.

Fig. 42.—Living Anthozoan Coral (Astræa).

In their general plan, the oldest Corals were precisely of this character, but they presented some differences in detail, which have caused them to be divided into two groups, which are eminently characteristic of the Palæozoic age—the tabulate or floored corals, and the rugose or wrinkled corals. In the former ([Fig. 43]) the cells are usually small and thin-walled, often hexagonal, like a honeycomb, and are floored across at intervals with tabulæ or horizontal plates. A few modern corals present a similar arrangement,[14] but this kind of structure was far more prevalent in the Palæozoic. In the second type the animals are usually larger and often solitary, the cell has strongly marked radiating plates, while the horizontal floors are absent or subordinate, and there is usually a thick external rind or outer coat ([Figs. 44, 45]). In general plan, these rugose corals closely resemble those of our modern reefs; but they differ in their details of structure, and only a very few modern forms from the deep sea are regarded as actual modern representatives.[15] One curious point of difference is that their radiating laminæ begin with four, and increase by multiples of that number, while in modern corals the numbers are six and multiples of six; a change of mathematical relation not easily accounted for, and which assimilates them to Hydroids on the one hand, and to a higher group, the Alcyonids, on the other, both of which prefer four and eight to six, or have had these numbers chosen for them. In the Mesozoic period the tabulate and rugose corals were replaced by others, the porous and solid corals of the modern seas; but, in so far as we know, the animals producing these, though differing in some details, were neither more nor less elevated than their predecessors, and they took up precisely the same rôle as reef-builders in the sea, though with probably more tendency to the accumulation of great masses of coral limestone in particular spots.

Fig. 43.—Tabulate Corals.

a, Halisites, and b, Favosites. Upper Silurian.

Fig. 44.—Rugose Coral (Heliophyllum Halli). Devonian.

Fig. 44a.—Zaphrentis prolifica (Billings). Devonian.

Fig. 45.—Rugose Corals.

a, Zaphrentis Minas (Dn.), and b, Cyathophyllum Billingsi (Dn.). Carboniferous.

Leaving the corals, we may turn to the sea-stars and seaurchins. These merely put in an appearance in the Early Cambrian, but become vastly multiplied in the Silurian, where the stalked feather stars (Crinoids) ([Fig. 46]) seem to have covered great areas of sea-bottom, and multiplied so rapidly that thick sheets of limestone are largely made up of the fragments of their skeletons. The ordinary star-fishes appear first in the Silurian ([Fig. 47]). The sea-urchins begin in the Upper Silurian, the early species having numerous and loosely attached plates, like some of those now found in the deep sea[16] ([Fig. 48]).

Fig. 46.—Modern Crinoid (Rhisocrinus Lofotensis).—After Sars.