326 THE GERM -PLASM 



their controlling forces will nevertheless be cumulative, and if 

 they are sufficiently numerous, they will determine the colora- 

 tion in question, and thus produce reversion. 



In this way we may, I think, account not only for the phenom- 

 enon of reversion in general, but also for many special details, 

 and more especially for the different degrees of reversion in 

 different races of pigeons. A sufficient number of experiments 

 has certainly not been made with regard to this point, but it is 

 nevertheless recognised that reversion occurs more easily and 

 more markedly in some races than in others. Darwin, for 

 instance, by crossing two black barbs with two red spots 

 obtained dark hybrids, of w^hich ' no less than six presented 

 double-wing bars.' * On the other hand, the mongrels derived 

 from two black barbs and two snow-white fantails showed no 

 trace of reversion. This must have been due to the retention 

 of a different number of unmodified specific determinants in 

 the germ-plasm of the two races, as well as to the difference of 

 the modified racial determinants ; for the more marked the 

 difference between the two crossed races, the more easily will 

 the ancestral determinants gain the predominance over them. 



The last-named experiment by Darwin w-as continued by 

 pairing two of these hybrids, one of which was brown and the 

 other black. The first brood (it is not stated how many there 

 were) displayed wing-bars ' of a darker brown than the rest of 

 the body.' This must have been due to the accumulation of a 

 larger number of unmodified determinants in individual germ- 

 cells in consequence of the reducing division, and to the subse- 

 quent union of two of these cells in the process of fertilisation. 

 We should therefore expect that reversion would not occur in 

 all the offspring of this pair, for the reducing division must also 

 result in certain germ-cells containing a majority of the modified 

 determinants. In the second brood of the same parents, in 

 fact, a brown bird was produced which possessed no trace of 

 wing-bars. 



It is easy on the basis of our theory to account for the fact 

 that a simple crossing of two species did not in many instances 

 produce any traces of reversion, although reversion resulted 

 from the subsequent double crossing. The most complete case 

 of reversion obtained by Darwin was produced as follows. A 



* Darwin, I.e., Vol.1., p. 208. 



