52 Causes of Emigrations 



Segregation 



Animals tend to segregate into groups which become subspecies 

 and in time species. It is generally believed that the ranges of 

 closely related subspecies and species are usually different (Jordan, 

 1905; Sumner, Osburn, and Cole, 1913; Sumner, 1932), but it is 

 not always so (Richards dC Robson, 1926; Pearse, 1933, 1934) . This 

 means that competing species usually are not closely related (Rob- 

 ertson, 1906) . A group of animals which can find an unoccupied 

 environmental niche escapes certain competitions that its ancestors 

 were obliged to endure. Though subspecies may intergrade along 

 the borders of their ranges, they may be so stable that they will 

 breed true for generations when isolated (Sumner, 1929) . New 

 races often originate at localities which are remote from those oc- 

 cupied by the parent stock (Sumner, 1928) . Two kinds of changes 

 may be said to take place in animals: (1) geographical, which are 

 often adaptive, and (2) constitutional, which also may be adaptive 

 or not (Crampton, 1925) . 



In their relations with each other, animals may be individualistic 

 or socialistic, as individuals or as species. They may compete and 

 struggle against each other for things in the environment, or they 

 may cooperate in such a way as to help each other to survive in the 

 struggle for existence. When two species contest for a habitat one 

 may be better adapted to survive in it than the other and finally 

 dominate. A maladapted species may survive for a long time in a 

 habitat if it is without competition, but, when it competes with 

 species which are better fitted to survive under the given conditions, 

 it soon becomes extinct (Warming, 1909) . An animal to survive 

 must continually struggle to overcome environmental resistance, 

 and if its biotic potential is not sufficient to compensate for bio- 

 logical as well as physical-chemical environmental factors which are 

 unfavorable, it cannot survive (Chapman, 1931). 



Many animals are especially adapted to avoid competition. Rob- 



