106 SEX INHERITANCE 



female-producing spermatozoa contain one more 

 chromosome and are larger in consequence, as Zeleny 

 and Faust have shown, such sperm might travel up 

 the oviduct with less speed than the male-producing 

 sperm. Hence the sex ratios would be affected in 

 favor of the males. Furthermore, secretions in the 

 oviduct might act differently on the two kinds of 

 sperm, the age of the sperm might affect one kind 

 more than the other, etc. Such effects would be 

 expected to bring about deviations from the normal 

 ratios, but effects of these kinds can not fairly be 

 brought forward to disprove the hypothesis that the 

 X-bearing spermatozoa give rise to females and the 

 no-X-bearing (or Y) spermatozoa give rise to males. 

 It has even been suggested that external conditions 

 might so weaken or strengthen the X chromosome 

 that an X-bearing spermatozoon might produce a 

 male or that a no X-bearing sperm might produce a 

 female. If such effects can be produced they would 

 act, no doubt, in the way postulated. But there is a 

 large amount of evidence showing that factors are 

 not ordinarily altered by environmental influences. 

 Nevertheless there is no conflict here w^ith the sex 

 chromosome mechanism, only another one is imagined 

 to have the power to overthrow it. No adherent of 

 the chromosome theory would deny the theoretical 

 possibihty that at times external conditions may 

 at least overcome the usual effect of the sex factor 

 if not the sex factor itself, but the burden of proof for 

 such supposed reversal of the normal result lies with 

 those who maintain it. Proof, if it were forthcom- 



