4. We now come to the argument from distribution. This is one of the weakest of the indirect supports of the doctrine of evolution; but, as it is much relied upon, it must be stated with all the force that it is supposed to have. The facts that are relied upon are these: When we survey the whole surface of the globe, so far as it is known to us, we find, in the first place, that the areas which have similar conditions (of soil and climate), and sometimes, where the areas are nearly adjacent, are occupied by quite different faunas. On the other hand, it is said that areas remote from each other in latitude, and contrasted in soil and climate, are occupied by closely allied faunas. The inference drawn is, that there is no manifest predetermined adaptation of the organisms to the areas, or habitats, in which they are found, because we do not find that like organisms are universally or generally found in like habitats, nor very unlike organisms in very unlike habitats. The conclusion is, that the facts of distribution in space do not conform to the hypothesis of design. In other words, the different animals found in different regions were not specially designed for those regions, but some of them have extended into regions of a different character; and when the regions are very unlike there are not found very unlike organisms, but there is a general similarity, or a less extensive variety. There is said, also, to be another important fact, namely, that "the similar areas peopled by dissimilar forms are those between which there are impassable barriers; while the dissimilar areas peopled by similar forms, are those between which there are no such barriers." Hence is drawn the conclusion that "each species of organism tends ever to expand its sphere of existence--to intrude on other areas, other modes of life, other media."[88] A good deal of aid is supposed to be derived for this argument respecting animal life by analogies drawn from the vegetable kingdom; but I can not help thinking that there is much caution to be observed in formulating such analogies into a law of universal application, or into one that relates to the existence of animal organisms. The origin, the multiplication, and the spread of animals involve a principle of life, organization and development which is very different in some important respects from that which obtains in the vegetable world. But, without laying any stress upon this distinction, and without intending to deprive the argument for animal evolution of any aid which it can derive from such supposed analogies, I pass to the specific argument respecting animal distribution. The argument is this: Races of organisms become distributed over different areas, and also through different media. They are thrust by the pressure of overpopulation from their old into new habitats, and as they diverge more widely in space they undergo more and more modifications of structure, by reason of the new conditions on which they enter.