1916] 



Taylor: Beavers of Western North America 



473 



inhabit the same area, and that "Jordan's law" does not apply to 

 this class of Eehinoderms ; and further, that "physiological isolation" 

 in some form has been a more important factor than geographical or 

 bathymetrical isolation in the specific differentiation of Ophiurans. 

 This state of things stands, of course, in strong contrast with the 

 widespread isolation of related species and subspecies in birds, mam- 

 mals, and vertebrates generally (see p. 475). 



RELATION OF EVIDENCE TO SOME CURRENT THEORIES 



Do the facts of distribution which we have before us meet the 

 requirements of Darwin's hypothesis as to the origin of species 



Geoqraphic Areas 



Fig. 0. Diagram illustrative of species arrangement as we ought to find 

 it in nature according to Darwin's theory of differentiation in 

 the same place through the advantage of divergence. 



through natural selection in the same place through the advantage 

 of divergence? Do they accord with De Vries' theory of mutations, 

 according to which the parent species, after passing into a mutation 

 period, gives rise to one or more elementary species or retrograde varie- 

 ties, which may coexist with the parent stock in the same locality? 

 Do they fulfill demands of Romanes ' and Gulick 's physiological selec- 



