EMBRYOLOGY. 139 
history of vegetal and animal life in general. Let us illus- 
trate 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 Horse, 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 
inthe 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 Anoplothe- 
rium 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 Rumi- 
nant 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 
ja 
traced through the Hog, Hippopotamus, Sheep, Deer, etc.; 
