THE PHENOMENA OF REVERSION • 325 



to those of the ancestral form, so that no one of them, did it 

 control the ontogeny, could resuh in the development of a 

 rock-pigeon. This conclusion depends entirely upon the race 

 breeding true, for a germ-plasm which still contained individual 

 unmodified idants of the ancestral form would also necessarily, 

 or at any rate probably, once in a way, contain a majority of 

 ancestral idants at the same time, in consequence of the incessant 

 recombination of the idants in the reducing division and in fer- 

 tilisation ; and this must result in reversion to the ancestral form. 

 Such a reversion, however, never occurs in purely bred races, 

 but only when crossing takes place. 



The germ-plasm of a race of pigeons therefore consists, 

 according to my view, of a certain number of idants, each of 

 which represents the type of the race. The majority of the ids 

 of which each idant is composed must consequently virtually 

 contain this type ; or, expressed more accurately, the whole of 

 the racial determmants — as compared with those unmodified 

 determinants which may still be present — are in the 7najority in 

 all the ids of every idant. 



The fact that races breed true can in this way be thoroughly 

 explained. 



As regards reversion to the coloration and markings of the 

 wild pigeon, we must suppose that, in the process of artificial 

 selection to which the different races owe their origin, only just 

 as many ids have become completely transformed into racial ids 

 as were required to ensure the desired object, viz., the preserva- 

 tion of the racial characters. A larger or smaller number of 

 determinants in many or perhaps all the ids must have remained 

 unmodified in all, or at any rate in many, of the idants. The 

 determinants on which the coloration depends, must have 

 remained unmodified in larger numbers than did those relating 

 to any other characters, for the coloration is the most liable 

 to revert. 



Reversion in the coloration will therefore occur when the 

 ancestral determinants for any particular region of the bird's 

 plumage gain a predominance over the racial determinants in 

 the course of development ; and in cross-breeds it will take 

 place when the racial determinants are so differetit that their 

 forces counteract one another instead of being cumulative. 

 Although the ancestral determinants are in the minority in the 

 grerm-cells of the two different breeds which unite at fertilisation. 



