castle: role of selection in evolution 383 



gene is plastic and yields readily to selection. Where only 

 genes of the former sort are involved, the principle of the pure 

 line is applicable ; where genes of the latter sort are involved, it 

 is not applicable. The divergent results obtained by Jennings 

 when dealing with Paramecium and when dealing with Difflugia 

 indicate that among asexually reproducing organisms, also, genes 

 are involved, some of which are stable, some of which are not. 

 Accordingly, what conclusion we reach as to the applicability of 

 the pure line theory in the breeding of animals and plants will 

 depend upon how common we find stable and plastic genes 

 respectively to be, and in what sorts of variations they are 

 involved. 



My own opinion, based upon a study through many years of a 

 variety of inherited characters in the smaller mammals, inclines 

 to the view that in such animals very few characters can safely 

 be referred to the agency of perfectly stable genes. Even in 

 color characters, probably the simplest as well as the most 

 studied of inherited characters, there is much fluctuation which 

 yields substantial results to selection by the discriminating 

 breeder. The yellows are not all of one shade, nor the blacks of 

 equal depth. The golden yellow of the Guernsey cow is very 

 different from the fawn of the Jersey or the dark red of the Devon. 

 Yet all are yellows, allelomorphs of black but each is selected for 

 a different standard to which the breeder must adhere very 

 carefully in his selections, if he wishes to win prizes or sell breed- 

 ing stock. 



When it comes to size and shape and that consistent inter- 

 relation of parts which the breeder calls "conformation," stable 

 genes cannot be detected. Crosses produce blends as regards 

 size and shape, and conformation is completely dissipated by a 

 cross. That is why the breeder is so reluctant to resort to an 

 out cross unless he is engaged merely in meat or wool production 

 and is not attempting to breed to a type. Aside from color there 

 are very few valued economic characters in our domestic animals 

 which are not inherited after the manner of blends. 



Weight of carcass, quality of wool, milk production in cattle, 

 egg production in fowls — all these are blending characters which 



