THE AMERICAN NATURALIST [Vol. XLIII 



strictly accurate since it excludes the idea of a whole 

 genus or species or variety mutating at once. 



While on this subject, I may mention that Mr. Darwin 

 anticipated the doctrine of the mutationists to the effect 

 that "when the organization has once begun to vary, it 

 generally continues varying for many generations." 47 

 But as to variability having periods of activity Mr. Dar- 

 win's opinion seems to have been unsettled. In a letter 

 to Weismann, in 1872, he remarks on the strangeness 

 "about the periods or endurance of variability," 48 but 

 in a letter to Moritz Wagner, in 1876, he says : 



Several considerations make me doubt whether species are much more 

 variable at one period than at another except through the agency of 

 changed conditions. I wish, however, that I could believe in this 



Practically this is the dilemma of the mutationists of 

 the present day : they are not in a position to prove that 

 plants and animals have periods of mutation, but they 

 assume that it must be so, because the belief "removes 

 many difficulties. ' ' 



One of Darwin's perplexities, however, has been ex- 

 plained away, as I have already pointed out, by the dis- 

 covery that mutation is not confined to a single case out 

 of millions of individual forms, nor even to a single gen- 

 eration out of a long genetic line, but that, as in the case 

 of the CEnotheras (evening primroses), a whole genus 

 is likely to be in a mutating condition at the same time, 

 producing- from each of several species numberless indi- 

 vidual mutants, which are themselves often in a mutating 

 condition, the parent stock meanwhile remaining per- 

 fectly constant. Such has been the case with (Enothera 

 {Onagra) lamarckiana, which, while throwing off, since 

 it has been under scientific observation, in large numbers 

 not less than a dozen elementary species and retrograde 

 varieties, has bred true to its original type through at 

 least one hundred and sixteen years, although there is 



""Origin of Species," 6th ed., p. 5. 



48 "Life and Letters," 1886, Vol. III. D . IKS 



