GEOGRAPHIC ISOLATION 75 



independently. The isolated form is endemic. The longer the isolation 

 continues, the larger is the number of mutations which appear, and 

 the greater the divergence from forms in other regions. The systematic 

 value of these endemic forms—variety, subspecies, species, genus, 

 family — consequently makes possible a conclusion as to the duration 

 and completeness of the isolation. Thus of the forms which have 

 colonized Krakatoa since 1883, coming from the neighboring islands, 

 not one is endemic. The native fauna of the Hawaiian Islands, by 

 contrast, is almost entirely composed of endemic species, with nu- 

 merous endemic genera and even families. 



Physiologic isolation is most frequently due to mutations of the 

 generative organs of such a nature that two groups lose their mutual 

 fertility. Infertility has thus come to be a criterion of specific dis- 

 tinctness. When, however, the differentiation into distinct species is 

 due to geographic isolation, mutual fertility need not be lost. The 

 geographic races of the lion or of the zebra are completely fertile; 

 but this is also true of such completely distinct forms as the various 

 pheasants, the red deer and wapiti, and the European and American 

 bisons. 3 



These theoretic considerations on the role of geographic isolation 

 in the evolution of species are amply supported by the facts of animal 

 distribution. If we take a given species in a specified area as a starting 

 point, we do not find its nearest relative in the same district, nor in 

 a distant one, but in an adjacent area separated by some barrier. This 

 is Jordan's Rule* which seems to hold rather generally for expanding 

 dominant groups of vertebrates. The whole body of research on geo- 

 graphic variation which is now being carried out in detail for birds 

 and mammals and some groups of insects, continues to confirm this 

 rule, and for examples it is sufficient to refer to any systematic work 

 on a large scale, such as Hartert's Birds of the Palaearctic Fauna. 

 There are certain exceptions in which intimately related forms occupy 

 the same area. Thus Dunn 4 regards the four species of Jamaican tree 

 frogs of the genus Hyla as related stocks which have diverged into 

 species sorted primarily by size at transformation. Among fishes, the 

 gobiid Eviota and the blenniids Enneapterus and Salarias are repre- 

 sented by closely allied pairs of species in the coral reefs of Samoa. 5 

 This is not surprising, in view of the possibility of physiological isola- 

 tion. That these cases must be cited as exceptions speaks for the 

 high degree of importance of geographic isolation. 



* The use of the term "law" for such ecological generalizations does not seem 

 advisable. 



