Darwin, and After Darwin Volume I Part 6

Here, then, we leave the lower forms of Metazoa in their condition of permanent gastrulae. They differ from the transitory stage of other Metazoa only in being enormously larger (owing to greatly further _growth_, without any further _development_ as to matters of fundamental importance), and in having sundry tentacles and other organs added later on to meet their special requirements. The point to remember is, that in all cases a gastrula is an open sac composed of two layers of cells--the outer layer being called the ectoderm, and the inner the endoderm. They have also been called the animal layer and the vegetative layer, because it is the outer layer (ectoderm) that gives rise to all the organs of sensation and movement--viz. the skin, the nervous system, and the muscular system; while it is the inner layer (endoderm) that gives rise to all the organs of nutrition and reproduction. It is desirable only further to explain that gastrulation does not take place in all the Metazoa after exactly the same plan. In different lines of descent various and often considerable modifications of the original and most simple plan have been introduced; but I will not burden the present exposition by describing these modifications[15]. It is enough for us that they always end in the formation of the two primary layers of ectoderm and endoderm.

[15] The most extreme of them is that which is mentioned in the last foot-note.

The next stage of differentiation is common to all the Metazoa, except those lowest forms which, as we have just seen, remain permanently as large gastrulae, with sundry specialized additions in the way of tentacles, &c. This stage of differentiation consists in the formation of either a pouch or an additional layer between the ectoderm and the endoderm, which is called the mesoderm. It is probably in most cases derived from the endoderm, but the exact mode of its derivation is still somewhat obscure. Sometimes it has the appearance of itself const.i.tuting two layers; but it is needless to go into these details; for in any case the ultimate result is the same--viz. that of converting the Metazoon into the form of a tube, the walls of which are composed of concentric layers of cells. The outermost layer afterwards gives rise to the epidermis with its various appendages, and also to the central nervous system with its organs of special sense. The median layer gives rise to the voluntary muscles, bones, cartilages, &c., the nutritive systems of the blood, the chyle, the lymph, and the muscular tube of the intestine.

Lastly, the innermost layer developes into the epithelium lining of the intestine, with its various appendages of liver, lungs, intestinal glands, &c.

I have just said that this three or four layered stage is shared by all the Metazoa, except those very lowest forms--such as sponges and jelly-fish--which do not pa.s.s on to it. But from this point the developmental histories of all the main branches of the Metazoa diverge--the Vermes, the Echinodermata, the Mollusca, the Articulata, and the Vertebrata, each taking a different road in their subsequent evolution. I will therefore confine attention to only one of these several roads or methods, namely, that which is followed by the Vertebrata--observing merely that, if s.p.a.ce permitted, the same principles of progressive though diverging histories of evolution would equally well admit of being traced in all the other sub-kingdoms which have just been named.

In order to trace these principles in the case of the Vertebrata, it is desirable first of all to obtain an idea of the anatomical features which most essentially distinguish the sub-kingdom as a whole. The following, then, is what may be termed the ideal plan of vertebrate organization, as given by Prof. Hackel. First, occupying the major axis of body we perceive the primitive vertebral column. The parts lying above this axis are those which have been developed from the ectoderm and mesoderm--viz. voluntary muscles, central nervous system, and organs of special sense. The parts lying below this axis are for the most part those which have been developed from the endoderm--namely, the digestive tract with its glandular appendages, the circulating system and the respiratory system. In transverse section, therefore, the ideal vertebrate consists of a solid axis, with a small tube occupied by the nervous system above, and a large tube, or body-cavity, below. This body-cavity contains the viscera, breathing organs, and heart, with its prolongations into the main blood-vessels of the organism. Lastly, on either side of the central axis are to be found large ma.s.ses of muscle--two on the dorsal and two on the ventral. As yet, however, there are no limbs, nor even any bony skeleton, for the primitive vertebral column is. .h.i.therto unossified cartilage. This ideal animal, therefore, is to all appearance as much like a worm as a fish, and swims by means of a lateral undulation of its whole body, a.s.sisted, perhaps, by a dorsal fin formed out of skin.

[Ill.u.s.tration: FIG. 45.--Ideal primitive vertebrate, seen from the left side. (After Hackel.) _na_, nose; _au_, eye; _g_, ear; _md_, mouth; _ks_, gill-openings; _x_, notochord; _mr_, spinal tube; _kg_, gill-vessels; _k_, gill-intestine; _hz_, heart; _ms_, muscles; _ma_, stomach; _v_, intestinal vein; _c_, body-cavity; _a_, aorta; _l_, liver; _d_, small intestine; _e_, ovary; _h_, testes; _n_, kidney ca.n.a.l; _af_, a.n.u.s; _lh_, true or leather-skin; _oh_, outer-skin (epidermis); _f_, skin-fold, acting as a fin.]

[Ill.u.s.tration: FIG. 46.--The same in transverse section through the ovaries; lettering as in the preceding Fig.]

[Ill.u.s.tration: FIG. 47.--_Amphioxus lanceolatus_. (After Hackel.) _a_, a.n.u.s; _au_, eye; _b_, ventral muscles; _c_, body-cavity; _ch_, notochord; _d_, intestine; _do_ and _du_, dorsal and ventral walls of intestine; _f_, fin-seam; _h_, skin; _k_, gills; _ka_, gill-artery; _lb_, liver; _lv_, liver-vein; _m 1_, brain-bladder; _m 2_, spinal marrow; _mg_, stomach; _o_, mouth; _p_, ventral pore; _r_, dorsal muscle; _s_, tail-fin; _t_, aorta; _v_, intestinal vein; _x_, boundary between gill-intestine and stomach-intestine; _y_, hypobranchial groove.]

Now I should not have presented this ideal representation of a primitive vertebrate--for I have very little faith in the "scientific use of the imagination" where it aspires to discharge the functions of a Creator in the manufacture of archetypal forms--I say I should not have presented this ideal representative of a primitive vertebrate, were it not that the ideal is actually realized in a still existing animal. For there still survives what must be an immensely archaic form of vertebrate, whose anatomy is almost identical with that of the imaginary type which has just been described. I allude, of course, to _Amphioxus_, which is by far the most primitive or generalized type of vertebrated animal hitherto discovered. Indeed, we may say that this remarkable creature is almost as nearly allied to a worm as it is to a fish. For it has no specialized head, and therefore no skull, brain, or jaws: it is dest.i.tute alike of limbs, of a centralized heart, of developed liver, kidneys, and, in short, of most of the organs which belong to the other Vertebrata. It presents, however, a rudimentary backbone, in the form of what is called a notochord. Now a primitive dorsal axis of this kind occurs at a very early period of embryonic life in all vertebrated animals; but, with the exception of _Amphioxus_, in all other existing Vertebrata this structure is not itself destined to become the permanent or bony vertebral column. On the contrary, it gives way to, or is replaced by, this permanent bony structure at a later stage of development. Consequently, it is very suggestive that so distinctively embryonic a structure as this temporary cartilaginous axis of all the other known Vertebrata should be found actually persisting to the present day as the permanent axis of _Amphioxus_. In many other respects, likewise, the early embryonic history of other Vertebrata refers us to the permanent condition of _Amphioxus_. In particular, we must notice that the wall of the neck is always perforated by what in _Amphioxus_ are the gill-openings, and that the blood-vessels as they proceed from the heart are always distributed in the form of what are called gill-arches, adapted to convey the blood round or through the gills for the purpose of aeration. In all existing fish and other gill-breathing Vertebrata, this arrangement is permanent. It is likewise met with in a peculiar kind of worm, called _Balanoglossus_--a creature so peculiar, indeed, that it has been const.i.tuted by Gegenbaur a cla.s.s all by itself. We can see by the wood-cuts that it presents a series of gill-slits, like the h.o.m.ologous parts of the fishes with which it is compared--i. e. fishes of a comparatively low type of organization, which dates from a time before the development of external gills. (Figs. 48, 49, 50.) Now, as I have already said, these gill-_slits_ are supported internally by the gill-_arches_, or the blood-vessels which convey the blood to be oxygenized in the branchial apparatus (see below, Figs. 51, 52, 53); and the whole arrangement is developed from the anterior part of the intestine--as is likewise the respiratory mechanism of all the gill-breathing Vertebrata. That so close a parallel to this peculiar mechanism should be met with in a worm, is a strong additional piece of evidence pointing to the derivation of the Vertebrata from the Vermes.

[Ill.u.s.tration: FIG. 48.--_Balanoglossus_. (After A. Aga.s.siz.) _r_, proboscis; _h_, collar; _k_, gill-slits; _d_, digestive posterior intestine; _v_, intestinal vessel; _a_, a.n.u.s.]

[Ill.u.s.tration: FIG. 49.--A large Sea-lamprey (_Petromyzon marinus_), much reduced in size. (After Cuvier and Hackel.) A series of seven gill-slits are visible.]

[Ill.u.s.tration: FIG. 50--Adult Shark (_Carcharias melanopterus_).

(After Cuvier and Hackel.)]

[Ill.u.s.tration: FIG. 51.--Diagram of heart and gill-arches of a fish.

(After Owen.)]

[Ill.u.s.tration: FIG. 52.--One gill-arch, with branchial fringe attached. (After Owen.) H, Heart.]

[Ill.u.s.tration: FIG. 53.--Diagram of heart and gill-arches in a lizard. (After Owen.) The gill-arches, _a a' a''_, and _b b' b''_, are called aortic arches in air-breathing vertebrata.]

Well, I have just said that in all the gill-breathing Vertebrata, this mechanism of gill-slits and vascular gill-arches in the front part of the intestinal tract is permanent. But in the air-breathing Vertebrata such an arrangement would obviously be of no use. Consequently, the gill-slits in the sides of the neck (see Figs. 16 and 57, 58), and the gill-arches of the large blood-vessels (Figs. 54, 55, 56), are here exhibited only as transitory phases of development. But as such they occur in all air-breathing Vertebrata. And, as if to make the h.o.m.ologies as striking as possible, at the time when the gill-slits and the gill-arches are developed in the embryonic young of air-breathing Vertebrata, the heart is constructed upon the fish-like type. That is to say, it is placed far forwards, and, from having been a simple tube as in Worms, is now divided into two chambers, as in Fish. Later on it becomes progressively pushed further back between the developing lungs, while it progressively acquires the three cavities distinctive of Amphibia, and finally the four cavities belonging only to the complete double circulation of Birds and Mammals. Moreover, it has now been satisfactorily shown that the lungs of air-breathing Vertebrata, which are thus destined to supersede the function of gills, are themselves the modified swim-bladder or float, which belongs to Fish. Consequently, all these progressive modifications in the important organs of circulation and respiration in the air-breathing Vertebrata, together make up as complete a history of their aquatic pedigree as it would be possible for the most exacting critic to require.

[Ill.u.s.tration: FIG. 54.--Ideal diagram, of primitive gill-or aortic-arches. (After Rathke.) H, outline of heart. The arrows show the course of the blood.]

[Ill.u.s.tration: FIG. 55.--The same, modified for a bird. (After Le Conte.) The dark lines show the aortic arches which persist. A, aorta; _p_, pulmonary arches; SC, S'C', sub-clavian; C, C', carotids.]

[Ill.u.s.tration: FIG. 56.--The same, modified for a mammal. (After Le Conte.)]

[Ill.u.s.tration: FIG. 57.--A series of embryos at three comparable and progressive stages of development (marked I, II, III), representing each of the cla.s.ses of vertebrated animals below the Mammalia (After Hackel.)]

[Ill.u.s.tration: FIG. 58.--Another series of embryos, also at three comparable and progressive stages of development (marked I, II, III), representing four different divisions of the cla.s.s Mammalia.

(After Hackel.)]

If s.p.a.ce permitted, it would be easy to present abundance of additional evidence to the same effect from the development of the skeleton, the skull, the brain, the sense-organs, and, in short, of every const.i.tuent part of the vertebrate organization. Even without any anatomical dissection, the similarity of all vertebrated embryos at comparable stages of development admits of being strikingly shown, if we merely place the embryos one beside the other. Here, for instance, are the embryos of a fish, a salamander, a tortoise, a bird, and four different mammals. In each case three comparable stages of development are represented. Now, if we read the series horizontally, we can see that there is very little difference between the eight animals at the earliest of the three stages represented--all having fish-like tails, gill-slits, and so on. In the next stage further differentiation has taken place, but it will be observed that the limbs are still so rudimentary that even in the case of Man they are considerably shorter than the tail. But in the third stage the distinctive characters are well marked.

So much then for an outline sketch of the main features in the embryonic history of the Vertebrata. But it must be remembered that the science of comparative embryology extends to each of the other three great branches of the tree of life, where these take their origin, through the worms, from the still lower, or gastraea, forms. And in each of these three great branches--namely, the Echinodermata, the Mollusca, and the Arthropoda--we have a repet.i.tion of just the same kind of evidence in favour of continuous descent, with adaptive modification in sundry lines, as that which I have thus briefly sketched in the case of the Vertebrata. The roads are different, but the method of travelling is the same. Moreover, when the embryology of the Worms is closely studied, the origin of these different roads admits of being clearly traced. So that when all this ma.s.s of evidence is taken together, we cannot wonder that evolutionists should now regard the science of comparative embryology as the princ.i.p.al witness to their theory.

CHAPTER V.

PALaeONTOLOGY.

The present Chapter will be devoted to a consideration of the evidence of organic evolution which has been furnished by the researches of geologists. On account of its direct or historical nature, this branch of evidence is popularly regarded as the most important--so much so, indeed, that in the opinion of most educated persons the whole doctrine of organic evolution must stand or fall according to the so-called "testimony of the rocks." Now, without at all denying the peculiar importance of this line of evidence, I must begin by remarking that it does not present the denominating importance which popular judgment a.s.signs to it. For although popular judgment is right in regarding the testimony of the rocks as of the nature of a history, this judgment, as a rule, is very inadequately acquainted with the great imperfections of that history. Knowing in a general way what magnificent advances the science of geology has made during the present century, the public mind is more or less imbued with the notion, that because we now possess a tolerably complete record of the chronological succession of geological formations, we must therefore possess a correspondingly complete record of the chronological succession of the forms of life which from time to time have peopled the globe. Now in one sense this notion is partly true, but in another sense it is profoundly false. It is partly true if we have regard only to those larger divisions of the vegetable or animal kingdoms which naturalists designate by the terms cla.s.ses and orders.

But the notion becomes progressively more untrue when it is applied to families and genera, while it is most of all untrue when applied to species. That this must be so may be rendered apparent by two considerations.

In the first place, it does not follow that because we have a tolerably complete record of the succession of geological formations, we have therefore any correspondingly complete record of their fossiliferous contents. The work of determining the relative ages of the rocks does not require that every cubic mile of the earth's surface should be separately examined, in order to find all the different fossils which it may contain. Were this the case, we should hitherto have made but very small progress in our reading of the testimony of the rocks. The relative ages of the rocks are determined by broad comparative surveys over extensive areas; and although the identification of widely separated deposits is often greatly a.s.sisted by a study of their fossiliferous contents, the mere p.r.i.c.king of a continent here and there is all that is required for this purpose. Hence, the accuracy of our information touching the relative ages of geological strata does not depend upon--and, therefore, does not betoken--any equivalent accuracy of knowledge touching the fossiliferous material which these strata may at the present time actually contain. And, as we well know, the opportunities which the geologist has of discovering fossils are extremely limited, if we consider these opportunities in relation to the area of geological formations. The larger portion of the earth's surface is buried beneath the sea; and much the larger portion of the fossiliferous deposits on sh.o.r.e are no less hopelessly buried beneath the land. Therefore it is only upon the fractional portion of the earth's surface which at the present time happens to be actually exposed to his view that the geologist is able to prosecute his search for fossils. But even here how miserably inadequate this search has. .h.i.therto been! With the exception of a scratch or two in the continents of Asia and America, together with a somewhat larger number of similar scratches over the continent of Europe, even that comparatively small portion of the earth's surface which is available for the purpose has been hitherto quite unexplored by the palaeontologist. How enormously rich a store of material remains to be unearthed by the future scratchings of this surface, we may dimly surmise from the astonishing world of bygone life which is now being revealed in the newly discovered fossiliferous deposits on the continent of America.

But, besides all this, we must remember, in the second place, that all the fossiliferous deposits in the world, even if they could be thoroughly explored, would still prove highly imperfect, considered as a history of extinct forms of life. In order that many of these forms should have been preserved as fossils, it is necessary that they should have died upon a surface neither too hard nor too soft to admit of their leaving an impression; that this surface should afterwards have hardened sufficiently to retain the impression; that it should then have been protected from the erosion of water, as well as from the disintegrating influence of the air; and yet that it should not have sunk far enough beneath the surface to have come within the no less disintegrating influence of subterranean heat. Remembering thus, as a general rule, how many conditions require to have met before a fossil can have been both formed and preserved, we must conclude that the geological record is probably as imperfect in itself as are our opportunities of reading even the little that has been recorded. If we speak of it as a history of the succession of life upon the planet, we must allow, on the one hand, that it is a history which merits the name of a "chapter of accidents"; and, on the other hand, that during the whole course of its compilation pages were being destroyed as fast as others were being formed, while even of those that remain it is only a word, a line, or at most a short paragraph here and there, that we are permitted to see. With so fragmentary a record as this to study, I do not think it is too much to say that no conclusions can be fairly based upon it, merely from the absence of testimony. Only if the testimony were positively opposed to the theory of descent, could any argument be fairly raised against that theory on the grounds of this testimony. In other words, if any of the fossils. .h.i.therto discovered prove the order of succession to have been incompatible with the theory of genetic descent, then the record may fairly be adduced in argument, because we should then be in possession of definite information of a positive kind, instead of a mere absence of information of any kind. But if the adverse argument reaches only to the extent of maintaining that the geological record does not furnish us with so complete a series of "connecting links" as we might have expected, then, I think, the argument is futile. Even in the case of human histories, written with the intentional purpose of conveying information, it is an unsafe thing to infer the non-occurrence of an event from a mere silence of the historian--and this especially in matters of comparatively small detail, such as would correspond (in the present a.n.a.logy) to the occurrence of _species_ and _genera_ as connecting links. And, of course, if the history had only come down to us in fragments, no one would attach any importance at all to what might have been only the _apparent_ silence of the historian.

In view, then, of the unfortunate imperfection of the geological record _per se_, as well as of the no less unfortunate limitation of our means of reading even so much of the record as has come down to us, I conclude that this record can only be fairly used in two ways. It may fairly be examined for positive testimony against the theory of descent, or for proof of the presence of organic remains of a high order of development in a low level of strata. And it may be fairly examined for negative testimony, or for the absence of connecting links, if the search be confined to the larger taxonomic divisions of the fauna and flora of the world. The more minute these divisions, the more restricted must have been the areas of their origin, and hence the less likelihood of their having been preserved in the fossil state, or of our finding them even if they have been. Therefore, if the theory of evolution is true, we ought not to expect from the geological record a full history of _specific_ changes in any but at most a comparatively small number of instances, where local circ.u.mstances happen to have been favourable for the writing and preservation of such a history. But we might reasonably expect to find a general concurrence of geological testimony to the larger fact--namely, of there having been throughout all geological time a uniform progression as regards the larger taxonomic divisions. And, as I will next proceed to show, this is, in a general way, what we do find, although not altogether without some important exceptions, with which I shall deal in an Appendix.

There is no _positive_ proof _against_ the theory of descent to be drawn from a study of palaeontology, or proof of the presence of any kind of fossils in strata where the fact of their presence is incompatible with the theory of evolution. On the other hand, there is an enormous body of uniform evidence to prove two general facts of the highest importance in the present connexion. The first of these general facts is, that an increase in the diversity of types both of plants and animals has been constant and progressive from the earliest to the latest times, as we should antic.i.p.ate that it must have been on the theory of descent in ever-ramifying lines of pedigree. And the second general fact is, that through all these branching lines of ever-multiplying types, from the first appearance of each of them to their latest known conditions, there is overwhelming evidence of one great law of organic nature--the law of gradual advance from the general to the special, from the low to the high, from the simple to the complex.

Now, the importance of these large and general facts in the present connexion must be at once apparent; but it may perhaps be rendered more so if we try to imagine how the case would have stood supposing geological investigation to have yielded in this matter an opposite result, or even so much as an equivocal result. If it had yielded an opposite result, if the lower geological formations were found to contain as many, as diverse, and as highly organized types as the later geological formations, clearly there would have been no room at all for any theory of progressive evolution. And, by parity of reasoning, in whatever degree such a state of matters were found to prevail, in that degree would the theory in question have been discredited. But seeing that these opposite principles do not prevail in any (relatively speaking) considerable degree[16], we have so far positive testimony of the largest and most ma.s.sive character in favour of this theory. For while all these large and general facts are very much what they ought to be according to this theory, they cannot be held to lend any support at all to the rival theory. In other words, it is clearly no essential part of the theory of special creation that species should everywhere exhibit this gradual multiplication as to number, coupled with a gradual diversification and general elevation of types, in all the growing branches of the tree of life. No one could adopt seriously the jocular lines of Burns, to the effect that the Creator required to practise his prentice hand on lower types before advancing to the formation of higher. Yet, without some such a.s.sumption, it would be impossible to explain, on the theory of independent creations, why there should have been this gradual advance from the few to the many, from the general to the special, from the low to the high.

[16] For objections which may be brought against this and similar statements, see the Appendix.

+---+--------------------------+--------------------------------------- _Epochs and Formations._ _Faunal Characters._ C a --------------------------+--------------------------------------- i POST-PLIOCENE. Man. Mammalia princ.i.p.ally of living n Glacial Period. species. Mollusca exclusively recent.

o +--------------------------------------- z PLIOCENE, 3,000 feet. Mammalia princ.i.p.ally of recent genera o --living species rare. Mollusca very i modern.

c +--------------------------------------- MIOCENE, 4,000 ft. Mammalia princ.i.p.ally of living o families; extinct genera numerous; r species all extinct. Mollusca largely OLIGOCENE, 8,000 ft. of recent species.

T +--------------------------------------- e EOCENE, 10,000 ft. Mammalia with numerous extinct families r and orders; all the species and i most of the genera extinct. Modern a type Sh.e.l.l-Fish.

r y +--- --------------------------+--------------------------------------- LARAMIE, 4,000 ft. Pa.s.sage beds.

--------------------------+--------------------------------------- M CRETACEOUS, 12,000 ft. Dinosaurian (bird-like) Reptiles; e Chalk. Pterodactyls (flying Reptiles); s toothed Birds; earliest Snake; bony o Fishes; Crocodiles; Turtles; z Ammonites.

o +--------------------------------------- i JURa.s.sIC, 6,000 ft. Earliest Birds; giant Reptiles c Oolite. (Ichthyosaurs, Dinosaurs, Lias. Pterodactyls); Ammonites; Clam- and o Snail-Sh.e.l.ls very abundant; decline r of Brachiopods; b.u.t.terfly.

+--------------------------------------- S TRIAS, 5,000 ft. First Mammalian (Marsupial); 2-gilled e New Red Sandstone. Cephalopods (Cuttle-Fishes, c Belemnites); reptilian Foot-Prints.

o n d a r y --- --------------------------+--------------------------------------- P PERMIAN, 5,000 ft. Earliest true Reptiles.

a +--------------------------------------- l CARBONIFEROUS, 26,000 ft. Earliest Amphibian (Labyrinthodont); e extinction of Trilobites; first o Coal. Cray-fish; Beetles; c.o.c.kroaches; z Centipedes; Spiders.

o +--------------------------------------- i DEVONIAN, 18,000 ft. Cartilaginous and Ganoid Fishes; c Old Red Sandstone. earliest and (snail) and freshwater Sh.e.l.ls; Sh.e.l.l-Fish abundant; decline o of Trilobites; May-flies; Crab.

r +--------------------------------------- SILURIAN, 33,000 ft. Earliest Fish; the first Air-Breathers P (Insect, Scorpion); Brachiopods and r 4-gilled Cephalopods very abundant; i Trilobites; Corals; Graptolites.

m +--------------------------------------- a CAMBRIAN, 24,000 ft. Trilobites; Brachiopod Mollusks.

r y --- --------------------------+--------------------------------------- A ARCHaeEAN, 30,000 ft. z Huronian. Eozoon, (probably not a fossil).

o Laurentian. i --------------------------+--------------------------------------- c PRIMEVAL. Non-sedimentary.

I submit, then, that so far as the largest and most general principles in the matter of palaeontology are concerned, we have about as strong and ma.s.sive a body of evidence as we could reasonably expect this branch of science to yield; for it is at once enormous in amount and positive in character. Therefore, if I do not further enlarge upon the evidence which we here have, as it were _en ma.s.se_, it is only because I do not feel that any words could add to its obvious significance. It may best be allowed to speak for itself in the millions of facts which are condensed in this tabular statement of the order of succession of all the known forms of animal life, as presented by the eminent palaeontologist, Professor Cope[17].

[17] For difficulties and objections, see Appendix.

Or, taking a still more general survey, this tabular statement may be still further condensed, and presented in a diagrammatic form, as it has been by another eminent American palaeontologist, Prof. Le Conte, in his excellent little treatise on _Evolution and its Relations to Religious Thought_. The following is his diagrammatic representation, with his remarks thereon.

When each ruling cla.s.s declined in importance, it did not perish, but continued in a subordinate position. Thus, the whole organic kingdom became not only higher and higher in its highest forms, but also more and more complex in its structure and in the interaction of its correlated parts. The whole process and its result is roughly represented in the accompanying diagram, in which A B represents the course of geological time, and the curve, the rise, culmination, and decline of successive dominate cla.s.ses.