AN ANALYSIS OF THE EFFECT OF SELECTION. 43 



5. Plumage and Toe Characters in Poultry. 



We are referred to the observations of Bateson and Davenport 

 for these cases. In one instance it is stated that Davenport has shown 

 that ''imperfection of dominance" and of segregation are the rule in 

 poultry. The question of imperfection of dominance is not apropos 

 in this connection. As Castle has said, regarding another case : 



" . . . .if black is crossed with brown, the crossbreds are apt to develop 

 in their coats more brown pigment granules than do homozygous or pure 

 blacks. Nevertheless, we have no reason to question the entire purity of 

 the gametes, both dominant and recessive, formed by such cross-bred black 

 animals. It is the dominance, not the segregation, which is imperfect." 

 (Castle, 1911, p. 91.) 



That Fi results do not bear on the question has been shown by 

 Bateson (1909), who says with regard to polydactylous fowls: 



"It might be pointed out that when, as in these examples, the abnormal 

 result is clearly perceptible in Fj, no question arises as to the occurrence of 

 an imperfect segregation. The peculiarity is evidently zygotic, and is caused 

 either by some feature of zygotic organization, or by the influence of external 

 circumstances." (Bateson, 1909, p. 251.) 



Moreover, in any case involving irregularities in dominance, im- 

 perfect segregation in crosses between different breeds would be very 

 difficult to demonstrate. 



6. Merino Sheep. 



No reference to the data in this case are given. I have been unable 

 to discover anything more definite than a few general statements by 

 practical breeders regarding the effects of crossing Merinos. 



Bateson admits, in the passage quoted above, that this and the 

 next case "may be ascribed to the interference of a multitude of 

 factors." 



7. Fantail Pigeons. 



This case has been studied by Morgan (Morgan, Sturtevant, Muller, 

 and Bridges, 1915, p. 186). The fantail type did not reappear in the 

 comparatively small F2 generation, but individuals not far from the 

 fantail were obtained; and when the Fi hybrids were mated to fan- 

 tails, several of the offspring fell within the range of the fantail race. 

 Bateson's "failure of a parental type to reappear in its completeness 

 after a cross" is, then, scarcely applicable to this case. 



8 and 9. Dutch Rabbits and Cases in Sweet Peas. Fractionation. 



These are the specific cases cited as illustrations of Bateson's theory 

 of "fractionation" or "subtraction stages," of which he states that 

 'it is to be inferred that these fractional degradations are the con- 

 sequences of irregularities in segregation." In the case of the sweet 

 pea, Bateson has pointed out that white flowers and the extreme dark 



