44 AN ANALYSIS OF THE EFFECT OF SELECTION. 



flowers of the deep purple Black Prince were among the earliest varia- 

 tions to appear, while the intermediate forms have arisen later, as he 

 suggests by fractionation. It would seem to follow that they have 

 arisen in heterozygous forms, for otherwise the fact that the larger 

 variants appeared first would be of no significance. There is, I think, 

 no evidence to show that the later variations did actually arise in 

 heterozygous forms, either in sweet peas or in rabbits. These factors 

 are all inherited separately, and this fact would seem to rule them 

 out of consideration if one adopts the chromosome theory of inheritance 

 or if one appeals to multiple allelomorphs as evidence in favor of the 

 variability of genes. In short, we have no evidence regarding the 

 origin of these forms, and their present behavior seems to indicate 

 that they are not due to fractionation. The only evidence in favor 

 of such a hypothesis is the somatic appearance of the characters. 



10. FLOWERING TIME IN PEAS. 

 Castle (1916a, p. 324) has summarized this case as follows: 



"Hoshino (1) recognizes that gametic contamination results from cross- 

 ing early and late flowering varieties; (2) recognizes also that variation may 

 occur among the cross-bred families, as well as in different pure lines of the 

 uncrossed races, as regards the 'quality,' value, or potency of the same gene; 

 (3) although Hoshino does not refer to the fact, his observations show clearly 

 that genetic variation of a gradual or fluctuating sort occurs in at least one 

 of the varieties which he crossed. 



" . . . . What I want to suggest is that in these several agencies we 

 have a sufficient explanation of the variation observed in Hoshino's F2, F 3 , 

 and F 4 generations, without invoking a two-factor hypothesis (as Hoshino 

 has done), one factor being enough." 



Castle's argument is that a difference in one pair of genes is sufficient 

 to account for the result, if contamination be assumed; and that one 

 difference is a simpler assumption than two. I have argued here that 

 such an assumption is not simpler, unless we can find positive evidence 

 that contamination ever occurs. In the present case, then, we must 

 turn to the evidence that led Hoshino to suppose contamination to 

 have occurred. 



Hoshino crossed an early-flowering pea and a late-flowering one. 

 The FI was nearly as late as the late parent; F 2 , obtained by self- 

 fertilizing FI, approximated fairly closely to 3 late : 1 early, but the 

 two classes were somewhat more variable than the corresponding 

 parent varieties, and apparently overlapped slightly. Hoshino self- 

 fertilized 236 of these F 2 plants and obtained 46 families that he 

 classified as constant, i. e., supposedly homozygous. This is a fair 

 approximation to the 1 in 4 expected if two pairs of genes are respon- 

 sible for the result. Hoshino shows that two pairs of genes will, in fact, 

 account for most of the results obtained. There are certain facts not 

 thus accounted for, but Hoshino shows (p. 265) that "secondary" 



