270 THE AMERICAN NATURALIST [VOL. XLIU 



varieties of cereals consists only in the differences among 

 the constituents of the mixtures. ' ' Much of the improve- 

 ment of grains that was formerly obtained by continued 

 selection De Vries ascribes to the unconscious selection 

 of elementary species and the gradual improvement of an 

 originally mixed stock. Kimpau's rye, a stable race 

 obtained by gradual selection, is thus accounted for, but 

 the burden of proof is here on the part of the mutationists. 

 It is apparently not so easy to test the role of mutations 

 versus fluctuations in the improvement of species, because 

 if one should secure a stable race by the usual process 

 of selection, the mutationist might urge that, after all, 

 amid the confusion of seemingly fluctuating variability, 

 there were some mutations which escaped notice, and, 

 through the unconscious selection of these and their off- 

 spring, the stock was gradually purified and converted 

 into an improved stable form. 



Where ordinary varieties include "hundreds of ele- 

 mentary forms," separated by characters which in many 

 cases are so small that they * * may be scarcely perceptible 

 to the inexperienced eye," how is one to tell whether he 

 is dealing with mutations or ordinary fluctuations ? The 

 latter may be much greater in extent, and, as we have seen, 



\J there is no structural criterion by which a mutation may 

 be recognized. Crossing experiments give us no certain 

 test and we have therefore to fall back upon the criterion 

 of stability and class as mutations those variations which 



y /breed true from their first appearance. Here the oppor- 

 tunities for begging the question are excellent. If by 

 the ordinary process of selection a stable race is pro- 

 duced we can of course ascribe it to the unconscious choice 

 of one or more undetected mutations. To be sure, stable 

 races can be produced only on the basis of stable varia- 

 tions, and if this is all that the mutation theory neces- 

 sarily implies its divergence from Darwin's teaching is 

 not very wide. 



If now it should turn out that stability is a matter of 

 degree the last distinguishing feature of the mutation 



