Seventh—It is common to find a new group arising near the close of some geologic period when vast climatic changes were taking place. Such an incipient group almost regularly becomes the dominant group of the next period, presumably because it arose in response to the new conditions that accompanied the change from one period to another.
Eighth—The evolution of the vertebrate classes is more satisfactorily shown than that of any other group, probably because it arouse within the period which is characterized by an abundant fossil record. Of the vertebrates, the mammals are best represented and show the most complete fossil pedigrees; this, because they are the most recent in origin and their remains have been least disturbed.
Ninth—Many practically complete fossil pedigrees have been worked out, connecting specialized types with simpler and more generalized ancestors. Such pedigrees have been worked out for the horse, the elephant, the camel, the rhinoceros and other equally specialized modern types. A single example of this type of evidence will be given, that of the horse. Many other pedigrees have been worked out that are equally complete and no less significant.
Pedigree of the Horse
As recorded by Dendy, the course of evolution of the horse family (Equidae) "has evidently been determined by the development of extensive, dry, grass-covered, open plains on the American continent. In adaptation of life on such areas structural modification has proceeded chiefly in two directions. The limbs have become greatly elongated and the foot uplifted from the ground, and thus adapted for rapid flight from pursuing enemies, while the middle digit has become more and more important and the others, together with the ulna and the fibula, have gradually disappeared or been reduced to mere vestiges. At the same time the grazing mechanism has been gradually perfected. The neck and head have been elongated so that the animal is able to reach the ground without bending its legs, and the check teeth have acquired complex grinding surfaces and have greatly increased in length to compensate for increased rate of wear. As in so many other groups, the evolution of these special characters has been accompanied by gradual increase in size. Thus Eohippus, of lower eocene times, appears to have been not more than eleven inches high at the shoulder, while existing horses measure about sixty-four inches, and numerous intermediate genera for the most part show regular progress in this respect.
All of these changes have taken place gradually, and a beautiful series of intermediate forms indicating the different stages from Eohippus to the modern horse have been discovered. The sequence of these stages in geological time exactly fits in with the theory that each one has been derived from the one next below it by more perfect adaptation to the conditions of life. Numerous genera have been described, but it is not necessary to mention more than a few."
The first indisputably horse-like animal appears to have been Hyracotherium of the lower eocene of Europe. Another lower eocene genus is Eohippus, which lived in North America, probably having migrated across from Asia by the Alaskan land connection. In Eohippus the forefeet had four hoofed toes of nearly equal size, the homelogue of the thumb having been reduced to a vestige. In the hind foot the great toe had entirely disappeared and the little toe had been reduced to a splint bone. Then came Orehippus of the upper eocene, Mesehippus of the lower miocene, Prothohippus of the lower pliocene, pliohippus of the upper pliocene, and finally, Equus of the quarternary and recent. This history, in so far as it concerns the characters already described, furnishes all of the intermediate conditions and perfectly connects the horses of the past with those of the present. One could hardly ask for a
