REPRODUCTION AND HEREDITY 125 



that an approximately equal number of Dd and dd conjuga- 

 tions may be expected. 



It was this phenomenon of the dominance of one of a 

 pair of alternative characters over the other (recessive) in 

 inheritance that led Mendel in his classical experiments 

 sixty- two years ago with garden peas, so to analyse the facts 

 of inheritance as to conclude that individual mature germ- 

 cells might carry the factor for one or the other alternative 

 character, but not the factors for both. The processes of 

 maturation, not observed by microscopists until long 

 afterwards, supply the mechanism by which these results 

 are brought about. One most important conclusion that 

 may be drawn from the family histories of these moths is, 

 that while the nature of the offspring may not always agree 

 with the nature of their parents (as in the family of a pair 

 of hybrid douhledayaria)^ it does depend on the nature of 

 the mature germ-ceils (gametes) which those parents carry. 

 This is one of many indications of the importance in 

 inheritance of the material which A. Weismann (1893, 

 1904) called '* germ-plasm," a material which we have 

 every reason to believe resides in the nuclear chromatin. 

 It seems at least clear that the factors carried by the chromo- 

 somes of the germ-cells determine certain characters of the 

 body. Many other examples, furnished by insects, of the 

 inheritance of alternative characters and of variation might 

 be given, and further reference to these important subjects 

 will be made in later chapters. There is, however, one 

 question which may be considered most suitably here, as it 

 concerns very closely the whole subject of reproduction ; 

 that question is the nature and determination of sex. 

 Great uncertainty still surrounds many aspects of the 

 problem, but some remarkable advances towards partial 

 solution have been made during recent years, and much of 

 our knowledge of sex- determination has been reached 

 through investigations on insects. 



In many insects as in other animals of which statistical 

 studies on the question have been made, the numbers of 

 males and females in a large population are approximately 



