THE EVOLUTION OF THE MAMMALIA. 229 
seems almost like a contradiction in terms. The series, with, 
perhaps, the exception of its earliest member, was entirely con- 
fined to North America ; it has been extinct since the end of the 
White River. 
Thus, of the three principal rhinoceros lines, only the main 
series acquired horns, the other two were hornless; each of the three 
series had a different and characteristic style of modification of 
the anterior teeth. 
The third family of perissodactyls, which has survived to the 
present day, is that of the tapirs. These animals were evidently 
derived from the same stock as the rhinoceroses, but separated 
from them at an early date, and were already distinct in the mid- 
dle Eocene. Owing to the wide geographical range of these 
early genera, it is not yet possible to decide in what part 
of the world they first originated. On the whole, the tapirs 
have undergone comparatively little change since their first 
recorded appearance, much less than either the horses or the 
rhinoceroses, being a relatively conservative and non-progres- 
sive type. The principal changes which are to be noted 
consist (1) in a gradual increase of size and weight, though no 
member of the family ever became very large ; (2) in a slow 
modification of the skull, to adjust it to the formation and 
increase of the proboscis ; (3) in slight, but peculiar changes in 
the anterior teeth and in the assumption by the premolars of the 
molar pattern. This gradual complication of the premolars 
until they come to resemble the true molars is a very character- 
istic feature of the perissodactyls, and is repeated in all the lines that 
were sufficiently long lived for the transformation to be com- 
pleted. 
Throughout the Tertiary period, the tapirs ranged all over the 
northern hemisphere, and in North America they continued abun- 
dant until nearly the end of the Pleistocene, entering South 
America with the great Pliocene migration. This former range 
explains their present remarkable distribution, which is in South 
America and southern Asia, with nearly half the circumference 
of the earth between them. The simple explanation is that they 
