574 THE AMERICAN NATURALIST. [VoL. XXXIX. 



This remarkable proportion of forms derived from the hybrids 

 AB, i. e., A, AB, and B in the ratio 1:2:1 can only be explained 

 on the assumption that the germ cells of the hybrid are pure 

 with respect to the characters of either one or the other of the 

 parents. The gametes from the hybrid, with the pure charac- 

 ters of either A or B and approximately equal in number, may 

 unite with one another in three possible combinations AA, AB, 

 or BB forming three types of offspring, one pure A, another 

 mixed AB, and the last pure B. By the law of chance the pro- 

 portions of these combinations ( AA, AB, and BB) in a simple 

 case will be in the ratio i : 2 : i. This assumption of the purity 

 of the germ cells of hybrids has been found to conform with the 

 facts in a number of simple experiments where two characters 

 such as A and B were sharply contrasted. When one of the 

 characters in the hybrid is dominant and the other recessive the 

 ratio can be expressed as D : UR : R as i : 2 : i which is merely 

 a substitution of D and R for the characters A and B. 



There are of course many factors which tend to modify the 

 ratios as stated above and complicate the results. Thus the 

 normal number of gametes may be of varying vigor and mortal- 

 ity so that there will be proportionately more or less of one type 

 of fusion than is called for by the law of chance. Sometimes 

 the characters of the parents remain evenly balanced in the 

 hybrid and refuse to split up in the succeeding generations, 

 remaining in a stable union in the germ cells produced by the 

 hybrid. Such conditions prove exceptions both to the law of 

 dominance and to that of purity of the germ cells. From these 

 exceptions and particularly the last it is difficult to believe that 

 any large proportion of the germ cells is absolutely pure, i. e., 

 bearing only the pure characters of one parent or the other. 

 H'ovvever, there is much evidence from our knowledge of the 

 distribution of the chromosomes from one generation to the next, 

 that certain relations are possible in the separation of germ plasm 

 which approximate the ratios of Mendel's law and while rarely 

 giving absolutely pure germ cells nevertheless do make possible 

 a large proportion of relatively pure cells. 



Let us examine now the chromosome history as a possible 

 physical basis for the Mendalian principles. Such considerations 



