Heredity and Sex 63 



analysis upon gametic differentiation, just as the color of a 

 guinea-pig in a mixed race of blacks and whites depends 

 upon whether the gametes which unite to produce it carry 

 black or white. As the heterozygous black guinea-pig 

 forms black-producing and white-producing gametes in 

 equal numbers, so there is reason to think male-producing 

 and female-producing gametes are formed in equal numbers 

 by the parent, in many cases at least. But is it not possible 

 that there may exist individuals which produce the two 

 sorts of gametes in unequal numbers, and so would have a 

 tendency to produce more offspring of one sex than of the 

 other ? Perhaps so, though we have no evidence that such 

 a condition, if it does exist, is transmitted from one genera- 

 tion to another. On this point I made experimental 

 observations upon guinea-pigs, extending over a series of 

 years. Oftentimes I found an individual that produced 

 more offspring of one sex than of the other, but this was 

 probably due merely to chance deviations from equality. 

 I could get no evidence that the condition was inherited, 

 though the experiment was continued through as many 

 as seven generations, including several hundred offspring. 

 The essential difference between a female and a male 

 individual is that one produces eggs, the other sperm. All 

 other differences are secondary and dependent largely upon 

 the differences mentioned. If in the higher animals (birds 

 and mammals) the sex glands (i.e., the egg-producing and 

 sperm-producing tissues) are removed from the body, the 

 superficial differences between the sexes largely disappear. 

 In insects, however, the secondary sex-characters seem to 

 be for the most part uninfluenced by presence or absence 

 of the sex glands. Their differentiation occurs independ- 

 ently though simultaneously with that of the sex glands. 



