Mutations and Evolution . 
77 
as I am aware, been suggested for Drosophila. Probably the 
reason is that in Drosophila the evidence is clear that most of the 
mutations at any rate have had their beginning in the germ plasm 
at the time they make their external appearance. A great deal of 
ink would have been spared if it had been recognised that for 
plants as for animals, for CEnothera as for Drosophila , mutation is 
a process suigeneris, a “ spontaneous” disintegration or alteration of 
elements in the germ plasm which finds certain physical parallels 
or analogies in the behaviour of the atom of radium and other 
radio-active substances. To attempt to explain away mutations 
by assuming that nothing new has really appeared is tantamount 
to a denial of evolution. Zoologists are fortunate in being free 
from this bogey, and in being able to recognise that whatever 
effects crossing may sometimes have on germinal behaviour, 
mutations take place which have no direct reference to it. 
Some of the more intricate details connected with the 
Drosophila experiments will be referred to later. We wish here 
merely to point out the way in which the plant and animal studies 
on mutation are reacting on each other for their mutual benefit. 
The clear-cut conception of lethal factors derived from Drosophila 
has been a great advantage in replacing the more indefinite 
conceptions of “ sterility ” in CEnothera. Again, the duplication 
of a chromosome in CEnothera is paralleled in several features by 
the non-disjunction of the sex-chromosomes which sometimes 
occurs in Drosophila. 
