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 fovt-bones are still clearly distinguish- 
