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Disregarding some problematical five-toed forms like Phenacodus, the first type of undoubted relationship to modern horses is Hyracotherium, a little animal about three feet long that lived during the Eocene period of the Cenozoic epoch.
The complex heavy grinders of the horse and elephant, the sharp cutting teeth of the carnivores, and the cropping teeth of the grass eaters were all still to come. Phenacodus is a characteristic genus of the early Eocene, whose species varied in size from that of a bulldog to that of an animal a little larger than a sheep.
The student of early animal life finds in the little Phenacodus, which was scarcely bigger than a good-sized setter dog, the beginnings from which many forms have subsequently developed. This creature showed points of structure which to-day may be seen in such diversified animals as the dog, the horse, the rabbit, and the monkey.
It is not, of course, suggested that Phenacodus was the immediate ancestor of any of these. But there were no animals in those times more like these I have mentioned than was Phenacodus, and from forms like it in main features all of these other animals have since been derived, each species of animal having become adapted to one particular kind of life.
The preliminary stages he calls “Phylembryos,” because they bear to the possible outcome of their evolution the same relation that the embryo does to the perfect individual. Thus, Phenacodus may be regarded as the Phylembryo of the modern horse. It is quite conceivable that each of our modern species may have had an independent series of Phylembryos reaching back to the primitive cells.
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