GENERAL DISCUSSION. 91 



varies much in degree in the heterozygote. The median comb may be 

 reduced to 70 per cent of its normal length or it may not develop at all. 



The second case of imperfection of dominance is that of polydactyhsm. 

 Extra-toe mated to normal gives extra-toe in 73 per cent only of the off- 

 spring in the case of the Houdans. Any trace of 6 toes (on one or both feet) 

 is found in only 12 per cent of the hybrid offspring from a 6-toed Silkie 

 parent. Certainly dominance here is very like blending. 



The third case of imperfection of dominance is that of syndactylism. 

 No syndactyls were noticed in Fj. My first conclusion was that syndactyl- 

 ism is recessive; but later studies have shown that it is dominant and that 

 all matings of two syndactyl parents yield about 56 per cent syndactyl 

 offspring. 



Rumplessness gives an illustration of how dominance may be so weak 

 as to be absent altogether; so that from F, alone the erroneous conclusion 

 is drawn that it is recessive; indeed, in one strain, only faint traces of 

 the character made their appearance in successive generations. 



Finally, winglessness is a character which appears not to be inherited 

 at all. Nevertheless our experience with rumplessness leads us to suspect 

 that winglessness also is an impotently dominant character. 



Looking at the matter frankly and without prejudice, the question 

 must be answered: Has not the whole hypothesis of dominance become 

 redudio ad ahsurdumf What visible criterion of dominance remains, where 

 dominance fails completely? All the usual statistical landmarks of pro- 

 portional appearance in successive generations being lost, can one properly 

 speak of dominance and recessiveness at all? 



Amid the general ruin of criteria, however, one means of detecting 

 dominance remains. That extracted character which in F^ or subsequent 

 generations shows in homologous * matings in some families a wide range 

 of variability is dominant, while that extracted character which constantly, 

 in all homologous matings, shows no or very little variation is recessive. 



The reason for this difference in the inheritableness of the two condi- 

 tions is easy to understand on the principles enumerated in the last section. 

 A positive character has a real ontogeny. But, as we have seen, the develop- 

 ment of any character may be interrupted at any stage. Most aberrations 

 among organisms are due to a retardation or failure of normal development. 

 In human affairs we recognize this tendency in the terms " degenerates " 

 and "defectives" (constituting from 2 to 4 per cent of the population). 

 Indeed, there are few persons who are not defective in some physical or 

 psychical character. In cases where the commonest form of abnormality 

 is due to a development in excess it seems probable that a normal restraining 

 or inhibiting factor is defective or absent. On page 88 I tried to show how 

 common in ontogeny such restraining and inhibiting factors are. Since onto- 



*By homologous matings I mean those in which the germ -plasms of both parents are in the same condition with 

 reference to the tmit-character; t. e., both either possess it pure or \&ck it altogether. 



