Paleontology. 191 



linear series in structure as in time. Confining 

 attention to the teeth and feet, it will be seen from 

 the wood-cut on page 189 that the former grow 

 progressively longer in their sockets, and also more 

 complex in the patterns of their crowns. On the 

 other hand, the latter exhibit a gradual diminution 

 of their lateral toes, together with a gradual strength- 

 ening of the middle one. (See Fig. 83.) So that in 

 the particular case of the horse-ancestry we have a 

 practically complete chain of what only a few years 

 ago were "missing links." And this now practically 

 completed chain shows us the entire history of what 

 happens to be the most peculiar, or highly specialized, 

 limb in the whole mammalian class namely, that of 

 the existing horse. Of the other two wood-cuts, the 

 former (Fig. 84) shows the skeleton of a very early 

 and highly generalized ancestor, while the other is 

 a partial restoration of a much more recent and 

 specialized one. (Fig. 85.) 



On the other hand, progressive modifications of the 

 artiodactyl feet may be traced geologically up to the 

 different stages presented by living ruminants, in some 

 of which it has proceeded further than in others. For 

 instance, if we compare the pig, the deer, and the 

 camel (Fig. 82), we immediately perceive that the 

 dwindling of the two rudimentary digits has pro- 

 ceeded much further in the case of the deer than in 

 that of the pig, and yet not so far as in that 

 of the camel, seeing that here they have wholly 

 disappeared. Moreover, complementary differences 

 are to be observed in the degree of consolidation 

 presented by the two useful digits. For while in the 

 pig the two foot-bones are still clearly distinguish- 



