If, then, when classified, organisms fall into assemblages such that those of the same grade are but indefinitely equivalent; and if, where evolution is known to have taken place, there have arisen assemblages between which the equivalence is similarly indefinite; there is additional reason for inferring that organisms are products of evolution.
§ 126. A fact of much significance remains. If groups of organic forms have arisen by divergence and re-divergence; and if, while the groups have been developing from simple groups into compound groups, each group and sub-group has been giving origin to more complex forms of its own type; then it is inferable that there once existed greater structural likenesses between the members of allied groups than exists now. This, speaking generally, proves to be so.
Between the sub-kingdoms the gaps are extremely wide; but such distant kinships as may be discerned, bear out anticipation. Thus in the formation of the germinal layers there is a general agreement among them; and there is a further agreement among sundry of them in the formation of a gastrula. This simplest and earliest likeness, significant of primitive kinship, is in most cases soon obscured by divergent modes of development; but sundry sub-kingdoms continue to show relationships by the likenesses of their larval forms; as we see in the trochophores of the Polyzoa, Annelida, and Mollusca—sub-kingdoms the members of which by their later structural changes are rendered widely unlike.
More decided approximations exist between the lower members of classes. In tracing down the Crustacea and the Arachnida from their more complex to their simpler forms, zoologists meet with difficulties: respecting some of these simpler forms, it becomes a question which class they belong to. The Lepidosiren, about which there have been disputes whether it is a fish or an amphibian, is inferior, in the organization of its skeleton, to the great majority of both fishes and amphibia. Widely as they differ from them, the lower mammals have some characters in common with birds, which the higher mammals do not possess.
Now since this kind of relationship of groups is not accounted for by any other hypothesis, while the hypothesis of evolution gives us a clue to it; we must include it among the supports of this hypothesis which the facts of classification furnish.
§ 127. What shall we say of these leading truths when taken together? That naturalists have been gradually compelled to arrange organisms in groups within groups, and that this is the arrangement which we see arises by descent, alike in individual families and among races of men, is a striking circumstance. That while the smallest groups are the most nearly related, there exist between the great sub-kingdoms, structural contrasts of the profoundest kind, cannot but impress us as remarkable, when we see that where it is known to take place evolution actually produces these feebly-distinguished small groups, and these strongly-distinguished great groups. The impression made by these two parallelisms, which add meaning to each other, is deepened by the third parallelism, which enforces the meaning of both—the parallelism, namely, that as, between the species, genera, orders, classes, &c., which naturalists have formed, there are transitional types; so between the groups, sub-groups, and sub-sub-groups, which we know to have been evolved, types of intermediate values exist. And these three correspondences between the known results of evolution and the results here ascribed to evolution, have further weight given to them by the fact, that the kinship of groups through their lowest members is just the kinship which the hypothesis of evolution implies.
Even in the absence of these specific agreements, the broad fact of unity amid multiformity, which organisms so strikingly display, is strongly suggestive of evolution. Freeing ourselves from pre-conceptions, we shall see good reason to think with Mr. Darwin, "that propinquity of descent—the only known cause of the similarity of organic beings—is the bond, hidden as it is by various degrees of modification, which is partly revealed to us by our classifications." When we consider that this only known cause of similarity, joined with the only known cause of divergence (the influence of conditions), gives us a key to these likenesses obscured by unlikenesses; we shall see that were there none of those remarkable harmonies above pointed out, the truths of classification would still yield strong support to our conclusion.
CHAPTER V.
THE ARGUMENTS FROM EMBRYOLOGY.
§ 127a. Already I have emphasized the truth that Nature is always more complex than we suppose ([§ 74a])—that there are complexities within complexities. Here we find illustrated this truth under another aspect. When seeking to formulate the arguments from Embryology, we are shown that the facts as presented in Nature are not to be expressed in the simple generalizations we at first make.