GEOGRAPHIC DISTRIBUTION OF ANIMALS: CONTINENTS 273 



only upon the nature of the barrier itself but also upon the nature of the 

 animals concerned. In general, a species can surmount a barrier in one of 

 two ways: ( 1 ) by being adaptable to a variety of living conditions or (2) by 

 giving rise to new forms adapted to conditions unsuitable for the parent 

 species itself. 



Adaptability is a most valuable attribute. Animals possessing it can ex- 

 tend their ranges into regions which offer conditions of life differing from 

 those in the center of dispersal. Animals possessed of a generous measure 

 of this quality range widely and change but little in response to the vary- 

 ing habitats they enter. In this way the various species of Old World rats 

 and mice, for example, have achieved distribution as world-wide as that of 

 man himself. And man is the star example of a form able to surmount all 

 barriers to dispersal by virtue of adaptability. 



Lack of adaptability, on the other hand, hinders, when it does not pre- 

 vent, such dispersal. Furthermore, it may lead to extinction of a species in 

 its home area if conditions in that area change sufficiently. The geologic 

 record affords many examples of such extinctions. Between the extremes 

 are found intermediate degrees of adaptability, contributing to the varying- 

 desrees of success with which animals meet changing conditions, either "at 

 home'' or as the species attempts to extend its range into new areas. 



Animals which do not possess adaptability enabling them to live in a 

 wide variety of habitats may be able to solve the problem of invading new 

 and differing regions in a different manner. They may be able to give rise 

 to new forms capable of living under conditions which the original spe- 

 cies could not tolerate. This solution is a much more common occurrence 

 than is the possession of the high degree of adaptability or versatility just 

 discussed. Evidence that evolution of new forms has occurred is provided 

 by the observation that as animals have radiated out from their center of 

 dispersal they have frequently become modified in various ways so that 

 they are no longer identical with each other or with their ancestors. The 

 camels are a case in point. The llama, the Bactrian camel, and the Ara- 

 bian camel differ somewhat from one another, and each differs somewhat 

 from the camels which formerly inhabited North America, their center of 

 dispersal (Fig. 12.5). Such differences, superimposed on fundamental simi- 

 larities and correlated with the distribution of the forms concerned, offer 

 eloquent testimony of evolution. 



Continuous Ranges 



While tapirs and camels afford examples of marked discontinuity in geo- 

 graphic range, the distribution of many groups of animals is more or less 



