GENERAL ZOOLOGICAL FACTS AND THEORIES 285 



We may study inheritance by examining the germ cells, by 

 compiling statistics, or by the experimental method of cross- 

 breeding and subsequent analysis of the characters of the off- 

 spring. It is not possible in any case to predict what characters 

 will be found in the offspring of known parents, although in 

 certain instances, to be brought out later, this may be done with 

 some degree of accuracy. 



(7) The Effects of Isolation 



Most evolutionists believe that a necessary condition for the 

 formation of new species is the isolation, separation, or segre- 

 gation of certain individuals from others of their kind. This may 

 be due to either geographical or physiological causes. 



GEOGRAPHICAL ISOLATION. " In the case of geographic or topo- 

 graphic isolation the isolated group or groups of individuals are 

 actually in another region or locality from the rest of the species, 

 this being the result of migration, voluntary or involuntary " 

 (250, p. 234). We have shown (p. 284) that normally there is 

 an overproduction of animals, and that dispersal is necessary for 

 the preservation of life. " In regions broken by few barriers, 

 migration and interbreeding being allowed, we find widely 

 distributed species, homogeneous in their character, the mem- 

 bers showing individual fluctuation and climatic effects, but 

 remaining uniform in most regards. ... In regions broken by 

 barriers which isolate groups of individuals we find a great 

 number of related species, though in most cases the same region 

 contains a smaller number of genera or families. . . . Given 

 any species in any region, the nearest related species is not likely 

 to be found in the same region nor in a remote region, but in a 

 neighboring district separated from the first by a barrier of some 

 sort" (248). 



The fauna of islands show in a striking manner the effects of 

 isolation. Some of these i?lands, if not of volcanic origin and 

 their animals the descendants of individuals that were driven 



