1915] 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 elimination of zygotes of this 
phaenotype, which according to all 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 O. 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 
1. Mass mutation consists in the production of unexpectedly 
large numbers of mutations, in some cases amounting to 100 per 
cent of the progeny. 
