In the last place, we must give more than a passing consideration to some of the extinct types of animals that occupy the position of “links” between groups now widely separated by their divergence in evolution from the same ancestors.  Perhaps the most famous example is Archaeopteryx found in a series of slates in Germany.  This animal is at once a feathered, flying reptile, and a primitive bird with countless reptilian structures.  Its short head possesses lizard-like jaws, all of which bear teeth; its wings comprise five clawed digits; its tail is composed of a long series of joints or vertebrae, bearing large feathers in pairs; its breastbone is flat and like a plate, thus resembling that of reptiles and differing markedly from the great keeled breastbone of modern flying birds, whose large muscles have necessitated the development of the keel for purposes of firm attachment.  In brief, this animal was close to the point where reptiles and birds parted company in evolution, and although it was a primitive bird, it is in a true sense a “missing link” between reptiles and the group of modern birds.  Other fossil forms like Hesperornis and Ichthyornis, whose remains occur in the strata of a later date, fill in the gap between Archaeopteryx and the birds at the present time, for among other things they possess teeth which indicate their origin from forms like Archaeopteryx, while in other respects they are far nearer the birds of later epochs.  That these links are not unique is proved by numerous other examples known to science, such as those which connect amphibia and reptiles, ancient reptiles and primitive mammals, as well as those which come between the different orders of certain vertebrate classes.

In summarizing the foregoing facts, and the larger bodies of evidence that they exemplify, we learn how surely the testimony of the rocks establishes evolution in its own way, how it confirms the law of recapitulation demonstrated by comparative embryology, and how it proves that the greater and smaller divisions of animals have followed the identical order in their evolution that the comparative study of the present day animals has independently described.

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The facts of geographical distribution constitute the fifth division of zooelogy, and an independent class of evidences proving the occurrence of evolution.  This department of zooelogy assumed its rightful status only after the other divisions had attained considerable growth.  Many naturalists before Darwin and Wallace and Wagner had noticed that animals and plants were by no means evenly distributed over the surface of the globe, but until the doctrine of evolution cleared their vision they did not see the meaning of these facts.  As in the case of all the other departments of zooelogy the immediate data themselves are familiar, but because they are so obvious the mind does not look for their interpretation but accepts the facts