A THEORY OF HEREDITY. 



221 



guinea pigs are crossed the second generation is all black, but the third 

 generation has three black and one white (fig. 1). 



First generation. 



Second generation. 



(From Castle : Pop. Sci. Monthly.) 



Third generation. 

 Fig. 1. — ^Effect of crossing white and hlack guinea pigs. 



The white is pure and breeds true. Only one of the blacks will do 

 this, however, because the other two are hybrids like the second genera- 

 tion. There is an equal distribution of both dominant and recessive 

 elements, although the apjDarent relation is three dominant to one reces- 

 sive. The distribution follows the law of chance as illustrated by H. H. 

 jSTewman in his lectures at Ann Arbor on heredity, or by the use of 

 colored discs. Put equal numbers of black and white strips of paper 

 into a receptacle and draw out one in each hand noting each time what 

 is produced, whether two white, two black, one white in the left hand, 

 one black in the right, or one white in the right hand and one black in 

 the left. An equal number of each of the four possibilities will result 

 if this process is continued indefinitely. However, Mendelism represents 

 something besides the law of chance, because the dominant obscures the 

 recessive, and the law holds true for individual cases, while only in the 

 ultimate result can the law of chance be tested. Mendelism may be the 

 method of readjustment of individual characters when they are brought 

 together by crossing after their extreme limit of variability has been 

 reached. 



Blending does not begin at once, but only after repeated crossing has 

 taken place. This may be explained by the action of the chromosomes 

 in the germ cells. Wlien the ovum has rid itself of unnecessary chromo- 

 somes and has conjugated with the spermatozoon to regain the chromatic 

 equilibrium of its kind (tjqje), then all the hereditary powers that produce 



