226 THE PERPETUATION OF ADAPTATIONS 



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 appeared. 

 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, (2d) red-eyed 

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

 along the lines of Mendelian segregation as is the case of sweet 

 peas. The sex chromosome (Fig. 101) may be represented by X, 

 the black X representing red eyes, the open X white eyes and 

 O no X. After maturation of the original white-eyed male one 

 type of spermatozoa has the open X, the other type has no X 

 as in the case of Pro tenor (Fig. 98). Fertilization affords an 

 equal chance for both types of spermatozoa. The egg cells 

 contain one black X. The Fi generation will all contain a black 

 X which is dominant over the open X and of course over no X. 

 When these are interbred, two black X's, a black X with an open 

 X, a black X with no X, and an open X with no X may result, 

 and the proportions are three red to one white. 



In this experiment only males appeared with white eyes, a 

 fact which might be interpreted as sex-limited inheritance males 

 only having white eyes. This however is not true for white- 

 eyed females may be produced by mating the above red-eyed 

 grand-daughters (containing a black X and an open X) with 

 the white-eyed male (containing an open X and no X) . Two 

 white X's are brought together and females with white eyes are 



