Foundations of the Mutation Concept. 23 



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 (Enothera. Again, the duplication 

 of a chromosome in (Enothera is paralleled in several features by 

 the non-disjunction of the sex-chromosomes which sometimes 

 occurs in Drosophila. 



