EXPERIMENTAL ZOOLOGY I95 



hundred generations we should find only gray animals in its 

 parentage until the first cross was reached, and yet not one 

 in the long chain has been a pure gray mouse. All have been 

 gray-whites, though in every generation some of the brothers 

 and sisters have been pure whites and pure grays. What then 

 becomes of the notion that an individual is of the pure breed 

 because for many generations it has bad only ancestors show- 

 ing the characters in question? Individuals are not neces- 

 sarily what they seem to be on the outside, for only by the 

 character of their offspring can their true nature be estab- 

 lished. 



How these ratios are accounted for by the assumption 

 that the gametes (eggs and spermatozoa) are in some way 

 carrying either the thing which makes grayness or whiteness, 

 but never both of these characters ; how it is that the germ 

 cells or gametes which are always pure with respect to the 

 gray or the white, while the adult individuals may be pure or 

 hybrid, I cannot discuss in the time available. Suffice it to 

 say that the conception of gametic purity is one of the funda- 

 mentals in Mendelism, and that modern studies in cytology 

 (the science of cellular biology) seem to point to the stage in 

 development at which the gametes of a hybrid animal may 

 receive one, but not both of a given pair of characters. 



The relation of grayness to whiteness in these mice is 

 only one example from a long list of such cases where Men- 

 del's law has been found to apply. It holds not only for 

 color characters but also for structural features like tallness 

 and dwarfness and, more curious still, for immunity and non- 

 immunity to some diseases, as in the case of wheat where im- 

 munity against rust behaves as a simple recessive to nonim- 

 munity, and it holds for man no less than for other animals. 

 A law of heredity which works in wheat and sweet peas and 

 in mice and human beings must be fundamental in its nature, 

 and such Mendel's law, or group of laws, appears to be. 



