The Method of Evolution 45 



fourths being black (Figs. 16-19). This result is explained 

 in the following way. The cross-bred black indi\idual 

 received from its black parent the character black (/i), 

 and from its white parent the character white {W). In it 

 according!)' black and white were associated together, but 

 only the former was manifested, the white remaining hidden 

 by the black. But in reproduction the cross-bred black 

 individual transmits black and white in separate cells. 

 And since these two kinds of cells are in the long run equally 

 numerous, it follows that the cross-bred black individuals 

 produce both black and white offspring in proportions fairly 

 constant (Fig. 20). 



Inheritance of this sort is called Mendelian, after Gregor 

 Mendel who first observed and explained it. The law 

 governing such inheritance is called Mendel's law. Such 

 inheritance is satisfactorily accounted for by the assump- 

 tion that the reproductive cells are at first dual in nature, 

 but become simple before they can function in the pro- 

 duction of a new individual. For this assumption we have 

 abundant evidence furnished by the direct study of the 

 reproductive cells with the microscope. These cells, like 

 the cells of the body in which they are contained, show in 

 their nuclei at cell division a fairly constant number of 

 bodies known as chromosomes. In the worm Ascaris there 

 are only 2 of these chromosomes; in the sea-urchin Toxo- 

 pneustes there are 36; in mice and men, about 24. 



A new individual arises, in sexually produced animals, 

 out of the union of an egg with a sperm. The sperm is 

 relatively small, but its influence equals that of the ngiuch 

 larger egg, which fact throws light on the nature of the 

 material basis of heredity. It suggests, namely, that 

 this material consists largel\' of ferment-like bodies which 



