EFFECTS OF INBREEDING ON THE SEX RATIO 31 



lism of the ova in such a way as to render the ova more easily fer- 

 tilized by one kind of spermatozoa than by the other. In the A 

 series of inbreds, under the conditions given, the ova tended to 

 attract spermatozoa that were 'male-producing;' in the B series, 

 the ova tended to attract spermatozoa that were 'female-pro- 

 ducing.' 



In advocating the possibility that fertilization may be selective 

 I am aware that I run counter to the general belief that any egg 

 is capable of fertilization by any spermatozoon that happens to 

 come in contact with it, and that those whose views have much 

 weight in molding biological opinion believe that this hypothesis 

 is ''so improbable as almost to invalidate any interpretation 

 into which it enters" (Wilson, '10). Just why this hypothesis 

 is considered so untenable is not clear. It is true that it has 

 not been definitely proved in any case, but neither has it been 

 disproved, nor has any convincing proof been offered, as yet, 

 for the very elaborate hypotheses that have been advanced to 

 account for heredity in general and in specific cases. The burden 

 of proof rests equally upon those who object to this hypothesis as 

 on those who maintain it. 



We owe to McClung ('02) the suggestion that the accessory 

 chromosome may be a sex-determinant. In discussing the pos- 

 sible action of the accessory chromosome in determining sex, 

 McClung ('02 a) states: 'even up to the time of fertilization the 

 female elements are so placed as to react readily to stimuli from 

 the mother. Here they are approached by the wandering male 

 elements from which they may choose — if we may use such a term 

 for what is probably chemical attraction — either the spermatozoa 

 containing the accessory chromosome, or those from which it is 

 absent. In the female element, therefore, as in the female or- 

 ganism, resides the power to select that which is for the best 

 interest of the species." 



In advocating selective fertilization as the probable cause of 

 anomalous sex ratios, Heape ('09) says: "it must be remembered 

 that there are an enormous number of spermatozoa available for 

 the fertilization of each ovum, and, moreover, it will be recol- 

 lected there are undoubtedly chemotactic properties associated 



