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.