The Mechanistic Conception of Life 21 



blindness in the next two generations corresponds quantita- 

 tively ^vith what we must expect on the assumption that the 

 chemical substances determining color vision are contained 

 in the sex chromosomes. In the color-blind individual some- 

 thing is lacking which can be found in the individual with 

 normal color perception. The factor for color vision is obvi- 

 ously transmitted through the sex chromosome. In the next 

 generation color blindness cannot appear, since each fertilized 

 egg contains the factor for color perception. In the second 

 generation, however, the theory demands that one-half of the 

 males should be color blind. In man these conditions cannot 

 be verified. T. H. Morgan has found in a fly (Drosophila) a 

 number of similar sex-limited characters which behave like 

 color blindness, e.g., lack of pigment in the eyes. These 

 flies have normally red eyes. Morgan has observed a muta- 

 tion with white eyes, which occurs in the male. When he 

 crossed a white-eyed with a red-eyed female all flies of the 

 first generation were red-eyed, since all flies had the factor 

 for pigment in their sex-cells; in the second generation all 

 females and exactly one-half of the males had red eyes, the 

 other half of the males, however, white eyes, as the theory 

 demands. 



From these and numerous similar breeding experiments of 

 Correns, Doncaster, and especially of Morgan, we may con- 

 clude with certainty that the sex chromosomes are the bearers 

 of those hereditary characters which appear pre-eminently in 

 one sex. We say pre-eminently, since theoretically we can 

 predict cases in which color blindness or white eyes must appear 

 also in the female. Breeding experiments have sho^\TL that this 

 theoretical prediction is justified. The riddle of Mendel's law 

 of segregation finds its solution through these experiments and 

 incidentally also the problem of the determination of sex which 

 is only a special case of the law of segregation, as ]Mendel 

 already intimated. 



