cox, DARWIN AND THE MUTATION THEORY 447 



doctrine of the mutationists to the effect that "when the organization has 

 once begun to vary, it generally continues varying for many generations." ^ 

 But as to variability having periods of acti\dty, Mr. Darwin'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," ^ 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 doctrine, as it removes many difficulties." ^ 



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 explained away, as 

 I have already pointed out, by the discovery that mutation is not confined 

 to a single case out of millions of individual forms, nor even to a single 

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

 (Enotheras (evening primroses), a whole genus is likely to be in a mutating 

 condition at the same time, producing from each of several species number- 

 less individual mutants, which are themselves often in a mutating condition, 

 the parent stock meanwhile remaining perfectly 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 considerable proof that it is itself a mutant from (Enothera grandiflora, 

 and none whatever for the assertion, often made, that it is a hybrid. As at 

 least nine of its mutants have also bred true through many generations in 

 pedigree cultures and doubtless had been constant forms for a long time in a 

 state of nature, there appears to be no ground for Darwin's fear that, grant- 

 ing the occurrence of mutation, the mutants would be liable to speedy 

 extermination through inability to propagate. Of course this would not 

 be the case with even a single self-fertilizing plant and it would not be true 

 with animal mutants if, like plant mutants, they were produced in numbers 

 by the mutating stock. As to sw^amping by intercrossing, it has been shown 

 that, under Mendel's law, in the extreme case of the production of a sohtary 

 mutant obliged to cross with the parent form, if it possesses characteristics 



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



2 " Life and Letters," 1886, Vol. Ill, p. 155. 



3 Ibid., p. 158. 



