40 



AN ANALYSIS OF THE EFFECT OF SELECTION. 



example. These exceptions may still be plausibly ascribed to the inter- 

 ference of a multitude of factors, a suggestion not easy to disprove; though it 

 seems to me equally likely that segregation has been in reality imperfect." 

 (Bateson, 1914.) 



Fractionation is referred to by Bateson in this same paper as prob- 

 ably due to imperfect segregation. Illustrations are Dutch rabbit 

 and Picotee and other sweet peas. (See p. 298.) 



"Accordingly we seem limited to the conclusion that a slowly blending 

 gene is involved in the cross between early flowering and late flowering peas, 

 that the blending after one generation of heterozygosis may be small in 

 amount, but after three generations it is in the majority of cases practically 

 complete, so that the commonest * constant ' class in the entire hybrid popula- 

 tion is one strictly intermediate between the modes of the parental varieties. 

 This interpretation is entirely in harmony with the obsei*ved modification 

 through crossing of many Mendelizing characters, as observed by Daven- 

 port, Bateson, and many others in poultry, guinea-pigs, swine, and other 

 animals, as well as in plants." (Castle, 19166, p. 215.) 



Hayes (1917) states on the basis of his experiments with variegated 



maize : 



*' . . . . One might conclude that certain heterozygous combinations 

 produce germinal instability which exhibits itself either as imperfect segrega- 

 tion, gametic contamination, or sporophytic variation." 



In these quotations the following cases have been cited as evidence 

 in favor of contamination, and therefore calling for investigation •} 



1. Poly dactyl guinea-pigs (Castle, 1906). 



2. Long-haired guinea-pigs (Castle and 



Forbes, 1906). 



3. Spotted guinea-pigs and rats (MacCurdy 



and Castle, 1907). 



4. English rabbits (Castle and Hadley , 1915) . 



5. Poultry, plumage and toe characters 



(Bateson and Davenport). 



Merino sheep. 

 Fantail pigeons. 

 Dutch rabbits. 



Picotee and other types of sweet peas. 

 Flowering time in peas (Hoshino, 1915). 

 Unspecified case in swine. 

 Variegated pericarp in maize (Hayes, 

 1917). 



Before we can discuss some of these cases inteUigently it is neces- 

 sary that we make sure what Castle means by the terms "gametic 

 purity" and ''unit-character." Unless these terms are understood 

 in such a way as to eliminate from consideration the idea of recombina- 

 tion of independent factors there is, of course, nothing to discuss. 

 If by gametic impurity or inconstancy of unit-characters is meant that 

 recombination of modifying factors occurs, the existence of such phe- 

 nomena must be granted at once — this is, in fact, the main contention 

 of the school of ''pure line" advocates or ''mutationists." I think the 

 two following quotations from Castle are sufficient to show that there 

 need be no disagreement on the question of defining these terms: 



"What we want to get at, if possible, is the objective difference between one 

 germ-cell and another, as evidenced by its effect upon the zygote, and it is 



^The rough-coated guinea-pig was foimerly cited (e. g., Castle and Phillips, 1914), but is now 

 never used. This is because Wright (Castle and Wright, 1916) has shown the results to be due 

 to multiple factors. 



