THE EVOLUTION OF THE MAMMALIA. 237 
reduced to small nodules, showing that the pad or cushion had 
now been developed. About this time began the migrations 
which led to the segregation of three quite distinct series of the 
sub-order. One series passed to the Old World and there gave 
rise to the true camels, the most ancient of which has been found 
in the Pliocene of India. The second series wandered to South 
America, where their descendants are the guanacos, llamas, etc., 
of the present time ; while the third remained in North America 
and are, on the whole, more like the South American than the 
Old World genera. Some of these North American species are 
very peculiar and one species, in the immense length of its neck 
and limbs, closely imitated the giraffe. Very large llama-like 
forms persisted here till well into the Pleistocene, when they all 
utterly disappeared, as did the horses, elephants, and many other 
strange types. 
Like the tapirs, the modern Tylopoda are discontinuously dis- 
tributed, and for precisely the same reason, because they have become 
extinct in the intervening regions, in which they formerly 
abounded. 
The history of the main line of the Tylopoda is thus almost as 
completely recorded and known as is that of the horses, and it is 
surprising to see how similar are the steps of evolution in these 
two widely separated groups. Both start from very small, five- 
toed ancestors, and in both we see a steady increase in stature, in 
the length of the neck, limbs, and feet, in the reduction of the 
external bones of the forearm and leg and of the digits. The 
reduction of digits culminates in the one-toed horse and the two- 
toed camel. The parallel might be carried into many of the 
minor details of structure, but the principle is sufficiently well 
illustrated. 
From this main line of tylopodan descent, many side branches 
were given off from time to time. Lack of space will prevent 
more than a very brief mention of some of the more important of 
these. Most of these branches, all of which are entirely pecul- 
iar to North America, first appear in the Uinta ; they become 
very abundant in the White River, somewhat less so in the John 
