III.
THE ARGUMENT FROM GEOLOGY.
But this allusion to fossils leads me to the next division of my subject—the argument from geology. It is not, however, necessary to say much on this head, for the simple reason that the whole body of geological evidence is for the most part of one kind, which although of a very massive, is of a very simple character. That is to say, apart from the increasingly numerous cases, such as the one just mentioned, which geology supplies of extinct “intermediate links” between particular species now living, the great weight of the geological evidence consists in the general fact, that of all the thousands of specific forms of life which palæontology reveals to us as having lived on this planet in times past, there is no instance of a highly organised form occurring low down in the geological series.[1] On the contrary, there is the best evidence to show that since the first dawn of life in the occurrence of the simplest organisms, until the meridian splendour of life as now we see it, gradual advance from the general to the special—from the low to the high, from the few and simple to the many and complex—has been the law of organic nature. And of course it is needless to say that this is precisely the law to which the process of descent with adaptive modification would of necessity give rise.
[1] Some of the lower vertebrata (Elasmobranch and Ganoid fishes) occur, indeed, in early strata (upper Silurian); but still far from the earliest in which some of the invertebrata are found. The general statement in the text applies chiefly to the more highly organised forms of the vertebrate series.
IV.
THE ARGUMENT FROM GEOGRAPHICAL DISTRIBUTION.
The argument from geology is the argument from the distribution of species in time. I will, therefore, next take the argument from the distribution of species in space—that is, the present geographical distribution of plants and animals. It is easy to see that this must be a most important argument, if we reflect that as the theory of descent with adaptive modification implies slow and gradual change of one species into another, and a still more slow and gradual change of one genus, family, or order into another genus, family, or order, we should expect on this theory that the organic types living on any given geographical area should be found to resemble or to differ from organic types living elsewhere, according as the area is connected or disconnected with other geographical areas. And this we find to be the case, as abundant evidence proves. For, to quote from Mr. Darwin, “barriers of any kind, or obstacles to free migration, are related in a close and important manner to the differences between the productions of various regions. We see this in the great difference in nearly all the terrestrial productions of the New and Old Worlds, excepting in the northern parts, where the land almost joins.... We see the same fact in the great difference between the inhabitants of Australia, Africa, and South America under the same latitude, for these countries are almost as much isolated from one another as possible. On each continent, also, we see the same fact; for on the opposite sides of lofty and continuous mountain ranges, of great deserts, and even of large rivers, we find different productions; though as mountain chains, deserts, &c., are not so impassable, or likely to have endured so long as the ocean-separated continents, the differences are very inferior in degree to those characteristic of distinct continents.” That is to say, the differences are usually confined to species and genera, whereas in the case of continents the differences extend to orders. Similarly in marine productions the same laws prevail—the species on the different sides of the American continent, for instance, being very distinct. Now, this law cannot be explained by any reasonable argument from design.
And still stronger does the present argument become when we look to the fossil species contained on different continents; for these fossil species invariably present the same characteristic stamp as the living species now flourishing on the same continents. Thus, in America we find fossils all presenting the characteristically American types of animals, in Australia the characteristically Australian types, and so on. That is to say, on every continent the dead species resemble the living species, as we may expect that they should, if they are all bound together by the ties of hereditary descent; while, if different continents are compared, the fossil species are as unlike as we have seen the living species to be.
Turning next to the case of oceanic islands, situated at some distance from a continent. In these cases the plants and animals found on the island, though very often differing from all other plants and animals in the world as regards their specific type, nevertheless in generic type resemble the plants and animals of the neighbouring continent. The inference clearly is, that the island has been stocked from the continent with these types—either by winds, currents, floating trees, or numerous other modes of transport—and that, after settling in the island, some of these imported types have retained their specific characters, while others have varied so as to become specific types peculiar to that island. The Galapagos Archipelago islands are particularly instructive in this connection; for while the whole group of islands lies at a distance of over five hundred miles from the shores of South America, the constituent islands are separated from one another by straits varying from twenty to thirty miles. Now, to quote from Darwin, “Each separate island of the Galapagos Archipelago is tenanted, and the fact is a marvellous one, by many distinct species; but these species are related to each other in a very much closer manner than to the inhabitants of the American continent.” That is to say, the American continent being some fifteen times the distance from these islands that they are from one another, emigration to them from the continent is of much more rare occurrence than emigration from one island to another; and therefore, as more time for variation is thus allowed, while the differences between the inhabitants of island and island are only specific, the differences between the inhabitants of the islands as a group and the inhabitants of the American continent are very often generic. I may mention, in passing, that it was upon discovering these relations in the case of the Galapagos Archipelago, and pondering upon them as “marvellous facts,” that Mr. Darwin was first led to entertain the idea that the doctrine of descent might be the grand truth for which the science of the nineteenth century was waiting.