1915] BARTLETT—MUTATION IN OENOTHERA IIr 
ratio of nummularia mutations to plants showed a variation roughly 
commensurate with the difference in germinability between the 
F, and F, seeds. In other words, the mortality among the year- 
old F, seeds appears to have been largely confined to seeds of 
typical O. pratincola. The ratio of nummularia mutations to seeds 
' planted is seen from table XII to be reasonably constant for all 7 
strains in both the F, and F, generations. The ratio of mutations 
to total plants, however, varies between wide limits, and in every 
case a low percentage of germination is associated with a high 
frequency of mutation. The F, progeny of Lexington A, for 
example, included 4 individuals of mut. nummularia among 255 
plants, a ratio of 1:64. These 255 plants, however, were obtained 
by sowing 1,083 seeds, of which only a small proportion (23.5 
per cent) germinated. There seems no escape from the conclusion 
that the percentage of germinable seeds of mut. mummularia had 
increased by virtue of the greater mortality among the seeds of 
typical O. pratincola. 
The evolutionary significance of differential mortality is too 
obvious to require any lengthy discussion. Mut. nwmmularia has a 
distinctly greater survival value than its parent when subjected 
to conditions which delay germination. It has already been shown 
that mut. nummularia has an enormously greater chance 
to survive than typical O. pratincola when subjected to certain 
unfavorable soil conditions. These facts should be carefully 
weighed by critics of the mutation theory who persist in assuming, 
as a matter of course, that mutations would have no chance to 
survive in competition with the more numerous typical plants. 
Dr Vries” has already shown that the percentage of mutation 
in a culture of O. Lamarckiana from seeds 5 years old was 40 per cent 
instead of the usual 6 per cent. In his comment on this remark- 
able result he states that in general the seeds of the mutation 
remain germinable longer than those of typical O. Lamarckiana, 
and suggests that it might be possible to make use of differential 
mortality to increase the proportion of mutations in seeds, and 
thereby to facilitate the discovery of the mutations. The writer 
unconsciously put this suggestion to a test at the time mutations 
* De Vrigs, H., Die Mutationstheorie 1:186. 1901. 
