MEXDELISM AND OTHER METHODS OF DESCENT 235 



to explain evolution. It is not too much to say that all who 

 have faced this question in the light of broad and thorough 

 knowledge of species in nature, agree with Darwin regarding 

 the generally continuous and the gradual character of evolution, 

 no matter how much they may differ with him regarding methods 

 and causes. It is at this point that taxonomic studies have a very 

 practical bearing on evolution, and become an almost indis- 

 pensable training and qualification for evolutionary judgment. 

 The difference between mutations and natural species is not that 

 they are less different than species, as Professor De Vries and 

 his followers had feared, but that they are more different than 

 species, and different in a different way. They lack the indis- 

 criminate diversity, the protean flexibility of form which often 

 renders it extremely difficult to find any completely diagnostic 

 differences between species which are nevertheless truly distinct 

 in nature. Mutations of the same species, on the contrary, 

 commonly differ in numerous, definite and much more constant 

 characteristics. The two phenomena, species and mutations, 

 belong to essentially distinct categories, as unlike as crowds and 

 drill-corps. 



Mutation is not a period, but a condition. A narrow-bred 

 group in the mutative condition all wears, as it were, the same 

 uniform. A new mutation constitutes a definite change in this 

 uniform. It would be very much easier to describe and classify 

 the military uniforms of the various European nations than to 

 find any equally diagnostic characters for the nations themselves. 

 The nations are much more seriously and essentially different, 

 of course, than their military uniforms, but the differences are 

 of a different sort, and not to be stated in the same terms. 

 Species have more essential evolutionary differences than muta- 

 tions, though mutations are at the same time more definitely 

 different. The phenomena are, in short, of "different categories 

 and incommensurable. Discussions can lead to no conclusions 

 until these fundamental diversities of facts and concepts are 

 appreciated. 



Professor Davenport has assured us that "The question of 

 the origin of species is that of the origin of specific characteris- 

 tics or differential marks." But, unfortunately, there is no evo- 



