118 THEORY OF EVOLUTION 



If, then, as these and other results to be de- 

 scribed point to the chromosomes as the bear- 

 ers of the Mendelian factors, and if, as will be 

 shown presently, these factors have a definite 

 location in the chromosomes it is clear that the 

 location of the factors in the chromosomes bears 

 no spatial relation to the location of the parts 

 of the body to each other. 



LOCALIZATION OF FACTORS IN THE 

 CHROMOSOMES 



The Evidence from Sex Linked Inheritance 

 When we follow the history of pairs of 

 chromosomes we find that their distribution in 

 successive generations is paralleled by the in- 

 heritance of Mendelian characters. This is best 

 shown in the sex chromosomes (fig. 57). In 

 the female there are two of these chromosomes 

 that we call the X chromosomes; in the male 

 there are also two but one differs from those of 

 the female in its shape, and in the fact that it 

 carries none of the normal allelomorphs of the 

 mutant factors. It is called the Y chromosome. 

 The course followed by the sex chromosomes 

 and that by the characters in the case of sex 



