THE MUTATION THEORY 



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Mass mutation differs from ordinary mutation only in 

 that the mutations, instead of being- produced in small 

 numbers, are produced in very large numbers. I\>r ex- 

 ample, the frequency of mutations in <K. Lamarck'tuna, 

 which shows ordinary mutability, is roughly l' per cent. 

 In certain strains of (E. Reynoldsii and (E. pratincola, on 

 the contrary, the number of mutations rises to 50 per 

 cent., or even 100 per cent., of the progenies. According 

 to Mendelian conceptions, it is impossible to get extracted 

 recessives in a progeny in excess of 33$ per cent., and in 

 order to get this many we must grant the elimination by 

 prohibition of the corresponding dominants. What shall 

 we say, then, of progenies containing 499 mutations out 

 of 500 plants, a condition which has actually been realized 

 in my cultures of (E. pratincola? It is impossible to in- 

 voke the elimination of a large class of typical plants, for 

 the typical zygotes are known to be stronger and better 

 fitted to develop than the mutational zygotes. My own 

 explanation is that most of the female germ cells of (E. 

 pratincola are a gametes and the male, /? gametes. The 

 phenomenon of mass mutation consists in the wholesale 

 production of modified a gametes, a', a", a'", a"", etc., each 

 of which corresponds to a different mutation and has 

 characters which impress a distinctive habit on the zygote 

 which is formed by fusion with an unmodified /? gamete. 

 In accord with this hypothesis the reciprocal crosses be- 

 tween mutation and parent species are mat roclinic. Mu- 

 tation pollinated with species gives mutation. Species 

 pollinated with mutation gives species. 



Mention has already been made of the mutations which 

 by self-pollination give progenies containing both the mu- 

 tational and the specific types. If the mutation is cross- 

 pollinated with pollen from the specific type, the progeny 

 is a mixture of two types, just the same as if self-pollina- 

 tion had occurred. On the contrary, if the specific type 

 is pollinated bv the mutation, only the specific type 

 occurs in the progeny. Here, it seems, we have a case 

 where the modification which results in the production of 

 a' instead of a gametes is reversible. Cases of this kind 



