Payne: Variations and Selection 



39 



These two hypotheses as I see them may be briefly stated 

 as follows: The mutationists hold that natural selection can 

 do nothing more than preserve favorable heritable variations. 

 They believe, in addition, that all heritable variations are 

 mutations. They further believe that selection can modify 

 a given character only by bringing in or getting rid of factors 

 which influence it, or by another mutation occurring in the 

 same direction in which selection is operative. This is the 

 multiple factor hypothesis. 



Castle's view, backed by his own experimental evidence 

 as he interprets it, is that fluctuating variations are expres- 

 sions of germinal changes and that these germinal changes 

 are variations of a single gene. 



Jennings has tried to bring these two different views 

 nearer together. First he uses the series of multiple allelo- 

 morphs of eosin eye color in Drosophila ampelophila. Here 

 are six variations or mutations of a single gene. Jennings 

 holds such variations comparable to Castle's hypothetical 

 fluctuating variations of the gene. Secondly he discusses the 

 seven modifiers of eosin eye color as described by Bridges 

 ('16), and which are located in chromosomes other than the 

 first. According to Jennings this set of modifiers of a single 

 character, in this case eosin eye color, could be used by the 

 adherents of the multiple factor hypothesis as an explana- 

 tion of the effect of selection. They might be brought to- 

 gether or separated in the course of selection. On the other 

 hand. Castle could use the six allelomorphs of eosin eye color 

 as an illustration of the variation of a single gene. Jennings 

 believes such differences are mere differences of expression 

 and that it matters not so far as selection is concerned 

 whether the variations which do occur are variations of a 

 single gene or of several genes. The essential fact is, accord- 

 ing to J ennings, that we have here two series of almost imper- 

 ceptible gradations in eye color, and he can see nothing more 

 for the selectionist to ask. The two viewpoints are essen- 

 tially one, except that Castle thinks of the changes as continu- 

 ous while the mutationists think of them as discontinuous. 

 However, this is again a question of detail according to 

 Jennings. 



Jennings meets one serious difficulty which, it seems to 

 me, he fails to meet squarely. This is the fact suggested by 



