igisl BARTLETT—MASS MUTATION 455 



zygosis by hybridization leads to such absurdities as the denial that 

 new forms have ever originated except by hybridization and 

 recombination. 



It is perhaps unwise to hazard even a guess at the nature of the 

 modification of the female gametes which results in mass mutation. 

 At one time the writer was inclined to believe that the modification 

 had involved the cytoplasm rather than the nucleus, and that cyto- 

 plasmic inheritance might account for the matroclinic crosses. 

 However, there are now adequate data at hand to show that similar 

 matroclinic crosses in other cases cannot be explained by cyto- 

 plasmic inheritance. The reason for discarding this hypothesis 

 will be explained in a future paper, since it involves data which 

 cannot be touched upon here. 



Mendelian expectations require that the largest class in a 

 progeny showing mutation shall consist of the parent phaenotype. 

 No explanation of the high mutability of mass mutating strains 

 can be accepted which requires the ehmination of zygotes of this 

 phaenotype, which according to aU other experience are strong and 

 viable. If a deficiency in any class of zygotes were to be expected 

 in a mass mutant strain, it would be the class of weakest mutations; 

 in the case of 0. pratincola, for example, it would be mut. setacea. 

 Yet this mutation is the very one which occurs in the largest 

 numbers. 



Mass mutation is neither more nor less easily explained than 

 ordinary mutation. It seems to be due to sudden mutative trans- 

 formations of certain female gametes, and to be apparent in the 

 zygotes without the necessity of subsequent segregation because 

 of the fact that the factors involved have no counterparts in the 

 male gametes. There is no real distinction between mass mutation 

 and ordinary mutation except that in the former type large num- 

 bers of gametes may be simultaneously affected, whereas in the 

 latter only a few are affected. 



Summary and conclusions 



I. Mass mutation consists in the production of unexpectedly 

 large numbers of mutations, in some cases amounting to 100 per 

 cent of the progeny. 



