CHAPTER X 



HEREDITY AND SEX. 



Evidence from Breedmg Experhne^its. Bryonia — Sex- 

 limited Heredity. The Horns of Sheep — Colour- 

 Blindness — Sex and Spurious Allelomorphism. The 

 Currant Moth — The Cinnamon Canary — The Silky 

 Fozvl — Aglia tau — Cytological Evidence — Summary. 



The facts of sexual dimorphism have to be considered 

 in any exposition of the laws of inheritance. There are two 

 aspects in which the phenomena of sex concern us: (i) the 

 nature and transmission of sex itself, (2) the influence which 

 sex has in deciding the development or suppression 

 of characters introduced into the zygote. The evidence 

 relating to these two questions is so closely interwoven 

 that they must be in practice treated together ; for the 

 facts concerning the influence of sex on the distribution of 

 characters constitute, as will be seen, a most important 

 means of indirectly investigating the problem of the actual 

 nature of sex, and have provided already several clues which 

 will probably lead to the unravelling of that bafliing mystery. 

 It need scarcely be said that in view of the great obscurity 

 still surrounding the genetics of sex any conclusions of a 

 positive kind can only be made tentatively. The phenomena 

 however are among the most interesting and important with 

 which the student of genetics is concerned, and every frag- 

 ment of evidence regarding them is at present worthy of 

 record. 



Numerous essays dealing with the subject of the de- 

 termination of sex ^ have recently appeared, and the belief 

 is extending that sex will probably be found to be a result 

 of gametic differentiation. This conclusion rests partly on 



* This chapter naturally makes no pretence to cover the whole ground. 

 I can treat only of those parts of the subject which come more immediately 

 within the scope of Mendel's principles. 



