284 THE AMERICAN NATURALIST [VOL. XLIII 



the same. In fact a survey of the racial differences of 

 man in their varying degrees and kinds and their correla- 

 tion with geographical distribution shows us pretty 

 clearly that these differences have been slowly acquired 

 by the summation of very small variations. These groups 

 are not related as the so-called retrograde or digressive 

 varieties are, but they are based on differences in general 

 constitution affecting the shape of the skull, the character- 

 istic complexion, the general temperament, and many 

 other traits too numerous to specify. They may have 

 arisen by minute, discrete, stable variations, but to call 

 each step in advance an elementary species seems absurd, 

 and to talk of the immutability of species still more so. 

 We gain little by characterizing as elementary species 

 the small steps of which there may be a dozen or more 

 separating a German from a Frenchman. 



Students of geographical distribution as a rule set 

 little store by the theory of mutation. The relation of 

 variation and species-forming to distribution as illus- 

 trated by the work of Gulick and Hyatt on the Achatin- 

 nelidae, the Sarasins on the snail fauna of Celebes, of 

 Plate on the mollusca of the Bahamas, and of many stu- 

 dents of the mammals, birds and fishes of North America 

 indicate that the steps concerned in species-forming have 

 been very modest ones. If sudden mutations of consid- 

 erable magnitude have been a not uncommon source of 

 varieties of domesticated animals and cultivated plants 

 it does not follow that the selection of comparatively 

 small variations has not been the predominant method 

 of species-forming in a state of nature. 



After fifty years from the publication of Darwin's 

 "Origin of Species" we are still debating, and more 

 lively than ever, the central problem of that epoch-making 

 book; but it is not improbable the views of its sagacious 

 author will prove more nearly correct than those of most 

 of his modern critics. Much remains to be done before 

 the problem is finally solved, and there are few fields 



