4o8 EVOLUTION, GENETICS, AND EUGENICS 



Ihemselves approximately fully after the complete removal of the 

 ovary at an early age." 



A beautiful experiment conducted by nature herself helps to drive 

 home the hormone theory of sex differentiation. In cattle, as shown 

 recently by F. R. Lillie, twins occur in a small percentage of cases and 

 involve the simultaneous fertilization of two eggs. These eggs lie as 

 a rule in opposite horns of the forked uterus, but owing to the growth 

 of their embryonic membranes the two individuals come to fuse cir- 

 culations so that there is an admixture of blood (Fig. 79). The result 

 is that if the twins are zygotically of the same sex no untoward effect 

 of blood admixture is apparent, but when the twins are zygotically a 

 male and a female, the female individual is always stopped in its 

 female differentiation and becomes more or less completely trans- 

 formed in a male direction. It appears, however, that at the time 

 when blood admixture occurs, the female individual has already 

 differentiated so far with respect to the external genitalia and in other 

 respects that, even though subsequent development be entirely male 

 in character, the resultant individual is always a sterile creature, 

 neither fully a female nor a complete male. Such individuals have 

 long been known as " freemartins." As a rare exception to the general 

 rule an occasional case has appeared in which a male and a female pair 

 fail to undergo blood admixture. In such cases both develop into 

 normal animals. It now appears that the reason why the female sex 

 is the one to suffer is that the male gonads differentiate precociously, 

 before the female, and inhibit the subsequent development of female 

 gonads. Hence the only hormones in the blood of both twins are 

 the male hormones. 



In conclusion we may say then that, in mammals, though 

 chromosomes tend to determine the primary sex differences, they 

 have no effect on the differentiation of secondary sexual characters. 

 These are due to substances secreted by the gonads that have been 

 called a hormones. 



SEX-LINKED HEREDITY 



Certain characters that are definitely heritable but have no obvious 

 relation to sex have been found to be more or less closely linked with 

 one or the other sex. A well-known instance of the kind of character 

 in question is color blindness in man. It has long been known that 

 color-blind individuals are almost invariably males, that such males 

 marrying normal women never have any color-blind offspring, but that 



