HEREDITY AND EVOLUTION 247 
Application of Mendelism. The laws of inheritance known as 
Mendelism were first discovered in plants. Afterward they 
were found equally applicable to animals, as well as to human 
beings. The value of definite laws which can be applied to 
animal and plant breeding can hardly be overestimated, while 
human society seems certain to be profoundly influenced by 
the application to itself of these same laws. 
Much greater progress has been made in the study of Mendel- 
ism in plants and animals than in man, as it is of course not 
possible to conduct experiments with human beings in the same 
way as with plants and animals. Moreover, we know that most 
human beings are very complex hybrids, and this in itself makes 
it difficult to study inheritance in man. Also, man is a slow- 
breeding animal, so that it has been impossible for modern 
students of heredity to observe many generations, while it is 
difficult to get accurate descriptions of characteristics of gen- 
erations that have died. 
Many human characters which seem to behave in Mendelian 
fashion have been catalogued. Among these are curly hair 
dominant over straight hair, dark hair over light hair, brown 
eyes over blue eyes, normal pigmentation over albinism, and 
normal condition over hereditary insanity or feeble-mindedness. 
In crosses between the negro and the white, color of skin 
does not act as a simple character dependent on one factor. In 
the #, and subsequent generations, however, there may be 
individuals much lighter or much darker in color than the 
mulatto parents. Indeed, the color may be as dark as that of 
a negro or light enough to pass for white. These results indi- 
cate that the color is conditioned by multiple factors which act 
in Mendelian fashion. In a previous connection we have seen 
that if three cumulative factors are concerned, then in the fis 
generation each of the original races would be represented by 
only one individual out of sixty-four, while sixty-two would be 
intermediate. If six factors were concerned, only one out of 
4096 individuals of the #, generation would have the germinal 
composition of each original race. 
