i8o 



Heredity and Environment 

 Flies Chromosomes 



XX Parents 



Gametes 



G&metea 



FIG. 61. SEX-LINKED INHERITANCE OF WHITE AND RED EYES IN Dro- 

 sopkila. Parents, white-eyed male and red-eyed female ; F lf red-eyed males 

 and females; F^ red-eyed females and equal numbers of red-eyed and 

 white-eyed males. The distribution of sex chromosomes is shown to right 

 of flies ; J carries the factor for red eyes, X the factor for white eyes, 

 stands for absence of X, or presence of Y chromosome which is found 

 only in males. (After Morgan.) 



is itself the result of antecedent causes and the cause of subse- 

 quent results. Undoubtedly the chromosomes are the results of 

 antecedent processes, and yet they may also be the causes of sub- 

 sequent results. No thoughtful person has ever maintained that 

 chromosomes or any other things in nature are autonomous, abso- 

 lute, uncaused causes. 



2. Linkage of Characters and Chromosomal Localization. 

 Finally the study of characters which are linked together in hered- 

 ity, joined with the study of the chromosomes and their distribu- 

 tion in the maturation and fertilization of the germ cells, has not 

 only confirmed the chromosomal theory of heredity but has also 

 shown that certain chromosomes carry the genes for certain char- 



