1 68 Heredity and Environment 



peared which cause the early death of those zygotes in which the 

 lethal gene is not balanced by a normal allelomorph. Some of 

 these lethals are sex-linked and alterations of the normal sex ratio 

 in certain cases may be explained as the result of these lethal 

 factors. Finally the chromosomal theory of sex determination is 

 so well supported in so many instances where at first it seemed 

 impossible of application, that it ought not to be rejected until 

 unmistakable evidences can be adduced against it. 



3. Hermaphrodites and Intersexes. Finally a number of cases 

 have been brought to light which indicate the necessity of dis- 

 tinguishing between the hereditary determination of sex and its 

 ontogenetic development. It has been known for a long time 

 that in bees and ants the workers are imperfect females, while 

 the queens are perfect females, and that the kind or amount of 

 food which is fed to the larvae determines whether they will be 

 workers or queens (see p. 233). Again in many animals the 

 development of male or female characters is dependent upon 

 internal secretions or hormones from the sex glands or other 

 organs (see pp. 234-237). In these cases it is evident that sex 

 was determined at an early stage, probably at fertilization, but 

 the development of these male or female characters, which occurs 

 later, is influenced also by the external or internal environment (p. 

 218). Breeders of cattle are familiar with the fact that when 

 twin calves are of opposite sex, the male is sexually perfect, but 

 the female usually has many male characters and grows into a 

 steer-like animal which is sterile and is known as a "free martin." 

 In a recent paper Lillie has shown that in all such cases the twins 

 are connected by blood vessels at an early stage in utero and that 

 there is a more or less complete circulation of blood from one 

 foetus to the other, and he concludes that "sex hormones," which 

 are probably formed earlier in the male than in the female, are 

 carried from the male to the female twin, thus causing the de- 

 velopment of male organs in an animal which would otherwise 

 have been a female. Therefore the chromosomal or "zygotic 



