history of vegetal and animal life in general. Let us illustrate by a few examples. We stated in the chapter on Zoology that the Horse, Tapir, and Rhinoceros formed a natural group, they being connected through intermediate forms, the series beginning with the Paleotherium (Fig. 151). In the chapter on Geology we called attention to the fact that the Paleotherium appeared before the Horse, etc. The embryo Plorse, however, in his three toes and the structure of his teeth, represents the Paleotherium (Fig. 154), while the transitory stages through which the Horse passes from the Paleotheroid condition to its adult state are permanently retained in the Anchitherium and Hipparion. (Fig. 155). In Ruminating animals, like the Gazelle, Sheep, and Ox, the upper jaw is without incisor and canine teeth (Fig. 157); these exist, however, in a rudimentary condition in the embryos of these animals. The embryos also exhibit two distinct metacarpal and metatarsal (Fig. 158) bones, which, in the course of development, fuse into the so-called cannon-bone of the fore and hind leg (Fig. 159). Now, in the early part of the Tertiary period there lived animals like the Dichobune, Dichodon, and Anoplotherium (Fig. 152), whose adult organization represents very well the transitory stage of the hollow-horned Ruminants, the Anoplotherium having well-developed canine and incisor teeth, and retaining the condition of two distinct metacarpal and metatarsal bones (Fig. 158). The stages through which one of our hollow-horned Ruminants passes give thus a picture of the transitional stages through which the Ruminant order in general has passed. The molar teeth of these animals, as well as those of the Rhinoceros, Horse, etc., are very interesting from the Evolution point of view. The type of tooth characteristic of the Paleotherium runs, more or less modified, through the Rhinoceros, Tapir (Fig. 153), and Horse, while that of the Anoplotherium can be traced through the Hog, Hippopotamus, Sheep, Deer, etc.;
