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THE AMEBIC AN NATURALIST [Vol. L 



to two factors which are completely linked in the true-breed- 

 ing single type and dissociated in the double-throwing single. 



2. As an alternative hypothesis to the above Frost suggests 

 that the pure-breeding single from which the double-throwing 

 single originated may not have been the same kind of single as 

 those which one meets with to-day, but that it differed from them 

 in one or other of the two ways suggested under 1. But again is 

 this a simpler explanation? 



Simplicity, however, though not to be disregarded, is not neces- 

 sarily the final test. Let us consider Frost's second argument 

 that his view can be supported by definite evidence. Failing to 

 find any positive histological evidence that can be taken as indi- 

 cating the required process of factor elimination, Frost suggests 

 that the case of the ever-sporting single is to be considered as 

 that of a hybrid showing selective sterility. To quote his own 

 words: 



Selective partial sterility seems to be rather a common phenomenon, 

 and it very probably occurs here. 



Functional sterility is to be supposed in the case of all pollen 

 grains carrying the factor (or factors) essential to singleness, 

 i. e., according to Frost's scheme, either S t — a modified form of 

 S, or S with a lethal factor linked to it. In other words, half 

 (we are to presume) of the pollen, though apparently good, is 

 supposed to be incapable of fertilizing the ovules. He further 

 adds that if we also assume 11 a slight tendency to selective elimi- 

 nation of S-carrying eggs" — a somewhat vague supposition to 

 account for a very definite fact — or if these egg cells were less 

 often fertilized than those which are s in constitution, or if there 

 were selective elimination of the embryos (Ss) that would pro- 

 duce singles, we should have "a simple and direct" explana- 

 tion of the constant excess of doubles. It is in fact suggested 

 that the postulated inability of the single-carrying pollen to 

 fertilize the ovules may be due to want of vigor. As bearing 

 on this point he calls attention to the greater vigor of the 

 double as compared with the ever-sporting single, as shown 

 in the vegetative habit of the growing plant, and in the greater 

 viability of the double-carrying embryos (seed). This greater 

 vegetative vigor of the ss over the Ss zygote Frost contends 

 may possibly be the outcome of a similar difference between 

 the s and S gametes. But if this were so, if the lesser vigor 

 of the ever-sporting single is due to the presence of one S fac- 



