92 INHERITANCE OF CHARACTERISTICS IN DOMESTIC FOWL. 



genetic processes are so often cut short by external conditions, we can under- 

 stand the variability in the degree of development of positive characters. 



On the other hand, whenever the fundamental hereditary stimulus or 

 the material for a character is absent from the germ-plasm of both parents, 

 then it can appear in none of the offspring; they will be practically invari- 

 able in respect to this condition. Only the ontogenetic fluctuations of other 

 real characters may influence the defect. Consequently the absent state 

 reproduces itself, the "recessive breeds true." 



The considerations here presented bear upon the hypothesis of change 

 of dominance. Bateson and Punnett (1905, p. 114) say of poultry: "The 

 normal foot, though commonly recessive, may sometimes dominate the 

 extra-toe character. " This idea of occasional change in dominance has been 

 expressed more than once in the literature. I think the phrase an unfor- 

 tunate one. In my earUer report * I urged that a characteristic that is 

 anywhere dominant is so without regard to race or species involved. If this 

 is so it is clearly improbable that it should vary from individual to individual, 

 or in the same individual at different times. Rather in view of the imper- 

 fection of dominance we should say that a dominant character sometimes 

 fails to develop, in which case it is absent from the progeny; that is all. 

 It is particularly apt to fail of development when dilute — heterozygous. 



C. POTENCY. 



Perhaps an apology is needed for introducing the much-abused word 

 "potency"; but there is hardly another that can be so readily adapted to 

 the precise definition I desire to give to it. The potency of a character 

 may be defined as the capacity of its germinal determiner to complete its 

 entire ontogeny. If we think of every character as being represented in the 

 germ by a determiner, then we must recognize the fact that this determiner 

 may sometimes develop fully, sometimes imperfectly, and sometimes not 

 at all. When such a failure occurs in a normal strain a sport results. 



Potency is variable. Even in a pure strain a determiner does not 

 always develop fully, and this is an important cause of individual varia- 

 bihty. But in a heterozygote potency is usually more or less reduced. 

 When the reduction is slight dominance is nearly complete; but when the 

 reduction is great dominance is more or less incomplete and, in the extreme 

 case, may be absent altogether. The series of cases of varying perfection 

 of dominance described in this work illustrate at the same time varying 

 potency. The extreme case is that of the rumpless fowl. The character in 

 this case is an inhibitor of tail development. This character has arisen 

 among vertebrates repeatedly and has become perpetuated in some amphibia 

 and primates, including man. In the case of our cock No. 117, the action 

 of the inhibitor is very weak, so that in the heterozygote the development 

 of the tail is not interfered with at all and even in extracted dominants it 



* Davenport, 1906, page 86. 



