i6 THE POPULAR SCIENCE MONTHLY 



chromosome. In the next generation color blindness can not 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 can not al- 

 ways be verified numerically since the number of children is too small 

 to yield the conditions to be expected according to the calculus of prob- 

 ability. 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. Mor- 

 gan has observed a mutation 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 conclude with certainty 

 that the sex-chromosomes are the bearers of those hereditary char- 

 acters which appear preeminently in one sex. We say preeminently 

 since theoretically we can predict cases in which color blindness or 

 white eyes must appear also in the female. Breeding experiments have 

 shown that this theoretical prediction is justified. The riddle of 

 Mendel's law of segregation finds its solution by 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. 



The main task which is left here for science to accomplish is the 

 determination of the chemical substances in the chromosomes which are 

 responsible for the hereditary transmission of a quality, and the deter- 

 mination of the mechanism by which these substances give rise to the 

 hereditary character. Here the ground has already been broken. It 

 is known that for the formation of a certain black pigment the coopera- 

 tion of a substance — tyrosin — and of a ferment of oxidation — tyrosinase 

 — is required. The hereditary transmission of the black color through 

 the male animal must occur by substances carried in the chromosome 

 which determine the formation of tyrosin or tyrosinase or of both. We 

 may, therefore, say that the solution of the riddle of heredity has suc- 

 ceeded to the extent that all further development will take place purely 

 in cytological and physico-chemical terms. 



While until twelve years ago the field of heredity was the stamping 

 ground for the rhetorician and metaphysician it is to-day perhaps the 

 most exact and rationalistic part of biology, where facts can not only be 

 predicted qualitatively, but also quantitatively. 



