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THE AMEBIC AN NATURALIST [Vol. XLIX 



possible " further distinct hereditary factor I merely 

 omit the sex-factor, and suppose the other factor to be 

 lethal in itself. He evidently does not notice that this 

 sort of factor might well explain more than the sterility 

 of the pollen. It amounts to the same thing, however, 

 to suppose the double-thrower S (or to be itself the 

 lethal factor. The introduction of sex-factors seems en- 

 tirely unnecessary here, and the supposed lethal elimina- 

 tion of an F factor can not be general in hermaphroditic 

 plants, since it would involve the universal occurrence of 

 sterile microspores or pollen. 



Our case appears to be merely one of a hybrid showing 

 selective sterility of pollen-grains, a sterility due to the S 

 factor or to a lethal factor linked with S. Further, if 

 there is also a slight tendency to selective elimination of 

 S-carrying eggs, we have a simple and direct explanation 

 of the excess of doubles over the expected 50 per cent. 

 Or, if the s-carrying eggs are more often fertilized, the 

 excess of doubles is explained. Once more, selective 

 elimination of single (Ss) embryos might produce the 

 same result. 



There are several facts which are extremely suggestive 

 in relation to all these possible forms of selective elimina- 

 tion. First, it is known that, in a double-throwing race, 

 the doubles are longer-lived than the singles in the seed 

 stage; Miss Saunders (1911, p. 362) has definitely con- 

 firmed the common belief that the proportion of doubles 

 tends to increase with the age of the seed. Second, Miss 

 Saunders (1911, p. 364) has obtained a higher proportion 

 of doubles from seed of lower viability, even with fresh 

 seed. Third, some seed-growers (deVries, 1906, p. 335) 

 regularly " starve " the seed-bearing plants, in the belief 

 that they thus increase the percentage of doubles among 

 the progeny. Fourth, the writer (Frost, 1911) has found, 

 with one variety, that inhibition of flowering by high tem- 

 perature is much more marked with singles than with 

 doubles; in field cultures, in many cases, hot weather 

 greatly delayed or entirely prevented flowering, the 



