230 THE PERPETUATION OF ADAPTATIONS 



essential factor in sex determination. In other cases, the evi- 

 dence is equally clear, but it is found that the sex chromosome is 

 not always alone, i.e. unpaired at maturation. In some cases, 

 its history may be easily followed because of some size difference 

 or other peculiarity. Thus in the bug Lygaeus, the sex chromo- 

 some of the male (X) , unites at synapsis with a smaller element 

 (Y), the end stages of maturation giving again two types of 

 spermatozoa, one type containing the sex chromosome (X) , the 

 other its smaller mate (Y) (Fig. 100). The female, on the other 

 hand, contains the full number of chromosomes including two 

 X's. After maturation there is one X in each egg. Fertili- 

 zation results in embryos with either one X or two X's. In the 

 former case, the individual is a male, in the latter case a female. 



In still other types of bugs, the corresponding X and Y 

 chromosomes are of equal size and cannot be distinguished 

 morphologically, but males and females are produced in equal 

 numbers, and the conclusion is justified that one of the chromo- 

 somes is the sex determining or X chromosome (see Ascaris, 

 Fig. 91). 



Experimental Evidence. The modern conception of sex deter- 

 mination, as outlined above, is beautifully supported by direct 

 experiments in breeding. It is quite conceivable, a priori, that 

 the sex chromosome X should contain factors standing for other 

 characteristics of the adult than sex alone. If this is true, then 

 certain peculiarities should appear only when the sex chromo- 

 some is present as a pure Mendelian segregation character. 

 An actual experiment will make this clear. Prof. Morgan has 

 carried out breeding experiments on the small fruit fly, Droso- 

 phila ampelophila, for several years. The wild fly has typical 

 red eyes, but during the experiments a white-eyed male ap- 

 peared. This was mated to a typical red-eyed female (Fig. 101). 

 The offspring were all red-eyed. These were then in-bred, and 

 the resulting brood contained (ist) red-eyed females, (2nd) 

 red-eyed males, and (3rd) white-eyed males, and in the propor- 

 tions of 50% of the ist, 25% of the 2nd and 25% of the 3rd, a 

 true Mendelian proportion. 



On the chromosome basis, this result is as easily explained 



