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THE AMERICAN NATURALIST [Vol. XLIX 



ence of a distinct linked lethal factor, makes this scheme 

 as complex as that of Miss Saunders ; this is not the case, 

 however, since Miss Saunders's scheme simply omits any 

 attempt at real explanation of the peculiarity of the 

 double-thrower pollen; her formulation imperatively re- 

 quires the addition of the hypothesis of selective via- 

 bility, or of some definite equivalent for it. s 



The real puzzle of the case lies in the fact that the 

 double-throwers plainly differ from the pure singles so 

 far tested in at least two respects— heterozygosity for 

 singleness (ability to form sporophylls) and the associa- 

 tion of some peculiarity with the remaining singleness. 

 This, however, is essentially a problem of the origin of 

 the double-throwing races, and is, in any case, nowhere 

 simpler than with the hypothesis here suggested. Miss 

 Saunders's scheme really implies four factorial or linkage 

 differences between pure singles and double-throwing 

 singles, and for certain cases six such differences, in place 

 of the two or three required by the hypothesis here 

 favored. That is, the double-thrower is supposed to 

 differ from the pure single in the following points: (1) 

 that it is heterozygous for two complementary factors 

 (X and Y) for which the pure single is pure, and in some 

 cases also for a second set of such factors (X' and Y') ; 

 (2) that its " singleness " can not be carried by func- 

 tional pollen; (3) that X and Y are partially instead of 

 completely linked. It is here proposed to drop half the 

 factors of (1), and this makes (3) superfluous. 



s She supposes (Saunders, 1911, p. 334) that X and Y are completely 



approximating, not the 25 per cent, assumed, but a slightly lower ratio. 

 A possible general slight deficiency of doubles in ttiis cross is not provided 



vigor. 



