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 gastræa, forms. And in each of these three great branches—namely, the Echinodermata, the Mollusca, and the Arthropoda—we have a repetition 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 mass of evidence is taken together, we cannot wonder that evolutionists should now regard the science of comparative embryology as the principal witness to their theory.
CHAPTER V.
Palæontology.
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 assigns 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 classes 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 assisted by a study of their fossiliferous contents, the mere pricking 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 shore 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 hitherto 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 palæontologist. 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 hitherto 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 analogy) 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 circumstances 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 palæontology, 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 anticipate 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 massive 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 assumption, 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.