80 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 
