172 HEREDITY AND ENVIRONMENT 



ally Morgan has found that there is a linkage 

 of certain somatic characters with sex in the 

 fruit fly, Drosophila, which can be readily ex- 

 plained by assuming that the determiners for 

 these characters are in some way associated 

 with the sex chromosome. By a series of the 

 most remarkable discoveries ever made in this 

 field he has shown that more than one hundred 

 somatic characters of Drosophila fall into four 

 groups corresponding in size and distribution 

 to the four chromosomes in the sex cells of this 

 animal, and not only is he able to locate the 

 factors for particular characters in particular 

 chromosomes but also he can determine the rel- 

 ative positions of many of the factors in each 

 chromosome. 



We have in these facts a remarkable corre- 

 lation between the distribution of the chromo- 

 somes and the occurrence of certain characters 

 of the adult animal. The association of ma- 

 ternal and paternal chromosomes in fertiliza- 

 tion and their segregation in the maturation of 

 the germ cells is parallel to the association of 

 Mendelian characters in the zygote and their 

 segregation in the gametes; if the distribution 



