314 



THE AMERICAN NATURALIST 



[Vol. XLI 



who maintain the truth of such a hypothesis to explain the 

 nature of this unknown mechanism. 



In the case of the accessory chromosome (McClung, :02), the 

 statement is made that it occurs in one half of the spermatozoa of 

 Orthoptera and Hemiptera. If we are to suppose that the acces- 

 sory chromosome acts as a sex determinant, and that sex characters 

 are to be treated as if they were Mendelian alternates (Wilson, 

 '07), we should find a sexual ratio equal to unity or differing from 

 unity by an extremely small per cent. We cannot, however, 

 account for the determination of sex in the human subject on any 

 basis of an equal division of spermatozoa into male and female 

 producing sperms, unless we suppose, as Wilson concedes for the 

 sake of argument, that sex may be modified by external condi- 

 tions. The statistical evidence is strongly against this alternative. 

 If it can be shown that the accessory chromosome occurs in the 

 spermatozoa of a species in the same proportion as the sex to which 

 it gives rise occurs in the young of that species, the statistical 

 evidence in its favor will be increased. At present, there is no 

 such evidence in its favor, as we do not know the exact sexual 

 ratio of the species in which the accessory chromosome has been 

 observed. 



The strongest evidence known to the author in favor of the 

 Mendelian theory of dominance in the determination of sex is 

 that cited by Harper (:07) in regard to plants. That the stamens 

 should develop and the pistil be suppressed in the fungus-infected 

 female plants of the campion is strongly suggestive of the recessive- 

 ness of the stamens under ordinary conditions. 



The application of Galton's law.— To explain the remarkable 

 constancy of the sexual ratio by Galton's law, we have only to 



is inherited equally from the paternal and nuiternal ancestry; 

 and to explain the preponderance of males in the itrcscnT uciit ia- 

 tion, we assume that in this ancestry for fiveor six u;(Mi(M;iri<iiis back, 

 there has been a preponderance of males, lii this wr arc justi- 

 fied since there is direct statistical evidence that, for more than 

 two hundred years, there has been an excess of male birtlis in 

 England. Accepting the statistics as hein<;- reasonably accm-ate, 

 the accordance with Galton's law of ancestral inheritance is much 

 closer than with Mendel's law. 



