74 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



time, and that the prevention of free crossing is an important element 

 in selective breeding. The development of the dairy breeds of cattle 

 on the islands of Jersey, Guernsey, and Alderney illustrates both of 

 these principles. The chief evidences of the importance of isolation in 

 the origin of species are derived from the study of the geographic 

 distribution of animals. Wagner 1 was the first to emphasize, from the 

 evidence of distribution, that geographic isolation must have an im- 

 portant effect upon the transformation of species and the formation 

 of varieties and subspecies. He even believed that this was the only 

 means by which one species can become differentiated from another, 

 failing to take into account the other means of preventing interbreed- 

 ing. The most important of the latter are, first, "physiological isola- 

 tion," brought about by a change in the bodily and instinctive char- 

 acters connected with reproduction itself, whereby the appearance of 

 distinct groups within a species is possible without any geographic 

 isolation, 2 and, second, ecological isolation which similarly enables 

 related forms to exist side by side in the same area though in different 

 habitats, or with different habits. 



The transformation of a species may take place by germinal 

 changes or mutation, either large or small, and by the selection through 

 environmental factors of such favorable mutations when they do 

 appear. Such mutational change takes place slowly. Geographic isola- 

 tion furthers the appearance of new adaptive modifications only when 

 the habitat conditions in the isolated area differ from those in the 

 original home of the species, but isolation may further the develop- 

 ment of new races based on non-adaptive characters even though the 

 habitat conditions do not differ essentially. Isolation, if it removes a 

 group of animals from enemies, may make possible the development 

 of structures or colors which would otherwise be eliminated. Somatic 

 transformation is directed into special channels under the influence of 

 the peculiar effects of climate, soil, food, and competition. 



The varying effectiveness of barriers in separating interbreeding 

 communities is of greatest importance to mutational evolution. 



If individuals from another area can enter a partially isolated 

 group, the newly acquired effective mutational changes will be brought 

 with them, and the differentiation of the group from the ancestral 

 form is thereby retarded, and this relation may be reciprocal. With 

 complete isolation, the groups differentiate first into varieties and sub- 

 species, and then, with longer periods of time, into wholly distinct 

 species and even into new genera and families. Such new forms are 

 confined to the isolated area in question, since it is in the highest 

 degree improbable that identical mutations should appear in a species 



