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. 



