GENERAL DISCUSSION. 93 



interferes little with tail development, so that it makes itself felt only in 

 reduced size of the uropygium and in bent or shortened back. But in No. 

 116 the inhibiting determiner is strong. It develops fully in about 47 per 

 cent of the heterozygotes and 2 extracted dominants may produce a family 

 in all of which the tail's development is inhibited. In the case of the rump- 

 less condition that arose apparently de novo in my yards, the new inhibitor 

 showed an intermediate potency completely stopping the tail development 

 in 1 out of 25 heterozygotes. These three cases afford a striking illus- 

 tration of a variation in the potency of the same inhibiting character in 

 different strains. 



Not only is potency variable, but its variations seem, in some cases, to 

 be inheritable. This we have seen to be the case with the Y-comb (p. 15) ; 

 with the extra-toed condition of Houdans (p. 23) ; and with rumplessness 

 (c/. offspring of No. 117 as compared with No. 116, p. 40). On the other 

 hand, the extra-toed condition of Silkies, the grade of clean shank, and the 

 degree of closure of nostril seem not to be inherited. 



D. REVERSION AND THE FACTOR HYPOTHESIS. 



The brilliant development of the factor hypothesis, only dimly fore- 

 shadowed by Mendel * (1866, p. 38), clearly expressed by Correns (1892), 

 applied to animals by Cuenot, and further elaborated by Bateson and Castle 

 and their pupils, has quite changed the methods of work in heredity. More 

 forcibly than ever is it brought home to us that the constitution of the 

 germ-plasm not merely the somatic character is the object of our investi- 

 gation. With this principle fully grasped the existence of cryptomeres and 

 the resolution of characters have become clearer. But the most striking 

 result accomplished has been that of clearing up the whole range of phe- 

 nomena formerly placed in the category of " reversion. " No idea without a 

 semblance of inductive explanation has been more generally accepted in 

 the Darwinian sense both by professed biologists and practical breeders 

 than this. Not only was the fact of recurrence of ancestral types in domes- 

 ticated organisms accepted, but the idea that, in some way, hybridization 

 per se destroyed the results of breeding under domestication was maintained. f 

 Now we know that, under domestication, many races have been preserved 

 that are characterized by a deficiency of a character or by a new, additional 

 one, and that hybridization, by bringing together again those characters 

 that are found in the ancestral species, may bring about again individuals 

 of the ancestral type. There is nothing more mysterious about reversion, 

 from the modern standpoint, than about forming a word from the proper 

 combination of letters. 



* Mendel's expression on this subject is translated by Bateson (1902, p. 84) as follows: " Whoever studies the colora- 

 tion which results in ornamental plants from similar fertilization can hardly escape the conviction that here also the 

 development follows a definite law which possibly finds its expression in the combination of several independent color charac- 

 ters, (The italics are Mendel's.) 



t"An inherent tendency to reversion is evolved through some disturbance in the organization caused by the art of 

 crossing.'* (Darwin, Animals and Plants under Domestication, Chapter XIII, section. "Summary on proximate causes 

 leading to reversion.") 



