60 H. J. MULLER 
Hoe 
tion. That this doctrine has been very widely accepted even 
among geneticists is indicated in the following quotations. 
‘The current view concerning the mutations of Oenothera is 
that they take place during synapsis, and that the sexual cells 
are in the mutated condition before the moment of self-fertiliza- 
tion’? (de Vries (18), p. 405). That de Vries probably does 
not intend this generalization to be limited to the peculiar 
‘mutations’ of Oenothera—which there is strong reason to 
believe are indeed phenomena of segregation—1is indicated by the 
following casual remark several pages later (p. 414): ‘‘ Assuming, 
as is now generally conceded, that mutations take place before 
fecundation . . . .’ In Babcock and Clausen’s (’18) text- 
book of genetics there appears a similar general statement 
(p. 269) which may probably be taken as representative of the 
current genetic opinion: ‘It would seem, therefore, that factor 
mutations in animals occur in the germ cells shortly before or 
during maturation.”’ ‘Tower’s more pronounced adherence to 
such a doctrine is well known. 
Although it has been shown above that mutations actually do 
occur at various stages in the life-cycle, yet when an attempt is 
made to obtain a quantitative estimate of the relative frequency 
of mutations at the different stages, much greater difficulties are 
encountered. The circumstances that must be taken into con- 
sideration in any investigation dealing with this subject may 
now be analyzed. In order to find out the time, in the life- 
cycle, at which a given mutant factor originated, it is usually’ 
necessary to know in how many members of a family the mutant 
factor first occurred concurrently. Ordinary recessive mutant 
factors, however, could not be thus detected at their first origin, 
since they would then be heterozygous and hidden (except under 
very exceptional circumstances), and so the inquiry must be 
limited to dominant factors and to sex-linked (dominant or 
recessive) factors, which are able to manifest themselves in the 
8 Except if the mutation is brought about by experimentally applied influ- 
ences, acting at a known stage in the life-cycle, or if the individual in which the 
mutation occurs is visibly a mosaic, as in the case of the new white, and i in the 
case of Baur’s doubtful mosaic mutations of Antirrhinum. 
