8 
NEW YORK ZOOLOGICAL SOCIETY. 
Similar radiations, on as large a scale, have taken place in early 
Eocene times among the marsupials in Australia, and somewhat 
later, in South America, which was then, and long afterwards, 
separated from North America, although probably for a shorter 
period connected with Africa and Australia, by way of Antarc- 
tica. In Australia the rapid replacement of the native Marsu- 
pials by the better equipped placental mammals, when the latter 
were introduced by man, is analogous to the manner in which 
the Puerco fauna was supplanted by the Wasatch fauna, through 
inability to successfully compete with those more highly organ- 
ized types. 
DECLINE OF ANCIENT GROUPS. 
After attaining a high degree of development this second or 
Wasatch fauna also declined, and one of the causes which con- 
tributed to its disappearance was the gradual elevation of the 
Western half of North America and the draining of the ancient 
lake basins there, with consequent loss of moisture. 
Desert lands, however, are not necessarily unfavorable to the 
development of structural variety and great bulk in animals. On 
the contrary, arid conditions seem to favor the development of 
large-hoofed animals, by imposing upon them the necessity of 
traveling over great stretches of country to find water during a 
drought. South Africa is a country of open plains, scantily sup- 
plied with water, many large areas being quite desert, and yet no 
other region of the earth can show such profusion of large mam- 
mals. 
As speed and endurance are to some extent correlated with 
bulk, no animal of small size could long survive competition 
with the faster and more enduring members of its own or a 
rival race. Increased size leads to an accelerated development, 
until a limit is imposed by the question of food supply. The 
larger the animal the more food it requires in proportion to its 
bulk, hence the larger animals must spend most of their 
time feeding, and if, through climatic changes, food becomes 
scarce, or enemies appear which they cannot resist, the larger 
animals are the first to succumb. 
A race, therefore, tends to increase in size until a maximum 
is reached, and is then apt to become suddenly extinct. The rhi- 
noceroses, elephants and horses are now on the verge of extinc- 
tion, all their smaller relatives having been weeded out by com- 
petition. 
