So THE POPULAR SCIENCE MONTHLY 



fishes, and to such reptiles as the herbivorous dinosaurs of the Upper 

 Cretaceous. 



This brief consideration of the external features of adaptation leads 

 us to glance at groups of animals. We here observe the influence of 

 geographic distribution; we observe the adaptive radiation of groups 

 both continental and local. 



Continental Adaptive Radiation. — Among the Tertiary mammals 

 we can actually trace the giving off of radii in several, sometimes in all, 

 directions for the purpose of taking advantage of every opportunity to 

 secure food, to escape enemies, and to reproduce kind, the three phe- 

 nomena of the struggle for existence. Among such well-known quad- 

 rupeds as the horses, rhinoceroses and titanotheres the modifications 

 involved in these radiations can be clearly traced. Thus the history of 

 the life of continents presents a picture of contemporaneous radiations 

 in different parts of the world. We observe the contemporaneous and 

 largely independent radiations of the hoofed animals in South America, 

 in Africa and in the great continent comprising Europe, Asia and 

 North America. 



Through the laws of parallelism and convergence each of these 

 radiations produced a greater or less number of analogous groups. 



While originally independent, the animals thus evolved separately as 

 autochthonous types in many cases finally mingled together as migrant 

 or invading types. 



We may thus work out gradually the separate contributions of the 

 great land masses of North America, South America, etc., to the mam- 

 malian fauna of the world. As a rule the greater the continents the 

 more important and fundamental the orders or larger groups of mam- 

 mals which have radiated in them; the lesser land masses and conti- 

 nental islands, like Australia, have been less favorable to wide adaptive 

 radiation. One of the most interesting features of adaptive radiation is 

 that it may also occur locally. 



Local Adaptive Radiation. — On a smaller scale are the local adaptive 

 radiations which occur through segregation of habit and local isolation 

 in the same general geographic region wherever physiographic and 

 climatic differences are sufficiently great to produce local differences in 

 food supply or other local factors of change. This principle is well 

 known among living animals, and it is now being demonstrated among 

 many of the Tertiary mammals, remains of four or five distinct genetic 

 series having been discovered in the same geologic deposits. 



The existence of multiple phyla of related animals, as of the rhi- 

 noceroses, horses and titanotheres in the same localities is due partly 

 to the operation of the law of local adaptive radiation. 



This is conspicuously the case among the titanotheres, for example, 

 the chief evolution of which can be traced in the Rocky Mountain 



