4 NUMBER OF SPECIES CHAP. I 
oceanic in origin. The continents and oceans are peopled by 
rather over three thousand species of Mammalia, a number which 
is considerably less than that of either birds or reptiles. It 
seems Clear that, so far at any rate as concerns the numbers of 
families and genera, the mammalian fauna of to-day is less varied 
than it was during the Mid- tertiary period, the heyday of 
mammalian life. It is rather remarkable to contrast in this way 
the mammals and the birds. The two classes of the animal 
kingdom seem to have come into being at about the same period ; 
but the birds either have reached their culminating point to-day, 
or have not yet reached it. The Mammalia, on the other hand, 
multiplied to an extraordinary extent during the Eocene and the 
Miocene periods, and have since dwindled. The break is most 
marked at the close of the Pleistocene, and may be in part due 
to the direct influence of man. At present man exercises so 
enormous an effect, both directly and indirectly, that the future 
history of the Mammalia is probably foreshadowed by the in- 
stances of the White Rhinoceros and the Quagga. On the other 
hand, the economic usefulness of the Mammalia is greater than 
that of any other animals; and the next most important era 
in their history will be probably that of domesticity and “ pre- 
servation.” 
