INTRODUCTION. 



A series of studies is here presented bearing on the question of dom- 

 inance and its varying potency. Of these studies, that on the Y comb 

 presents a case where relative dominance varies from perfection to entire 

 absence, and through all intermediate grades, the average condition being 

 a 70 per cent dominance of the median element. When dominance is rela- 

 tively weak or of only intermediate grade the second generation of hybrids 

 contains extracted pure dominants in the expected proportions of 1:2:1; 

 but as the potency of dominance increases in the parents the proportion 

 of oflfspring with the dominant (single) comb increases from 25 per cent to 

 50 per cent. This leads to the conclusion that, on the one hand, dominance 

 varies quantitatively and, on the other, that the degree of dominance is 

 inheritable. 



The studies on polydactylism reveal a similar variation of potency in 

 dominance and show, in Houdans at least, an inheritance of potency (table 

 11), and moreover they suggest a criticism of Castle's conclusion of inherit- 

 ance of the degree of polydactylism. 



Syndactylism illustrates another step in the series of decreasing potency 

 of the dominant. On not one of the F, generation was the dominant (syn- 

 dactyl) condition observed; and when these hybrids were mated together 

 the dominant character appeared not in 75 per cent but in from 10 per cent 

 to per cent of the offspring. The question may well be asked : What is 

 then the criterion of dominance? The reply is elaborated to the effect that, 

 since dominance is due to the presence of a character and recessiveness 

 to its absence, dominance may fail to develop, but recessiveness never can 

 do so. Consequently two extracted recessives mated inter se can not throw 

 the dominant condition; but two imperfect dominants, even though indis- 

 tinguishable from recessives, will throw dominants. On the other hand, 

 owing to the very fact that the dominant condition often fails of develop- 

 ment, two extracted "pure" dominants will, probably always, throw some 

 apparent "recessives." Now, two syndactyls have not been found that 

 fail (in large families) to throw normals, but extracted normals have been 

 found which, bred inter se, throw only normals; hence, "normal-toe" is 

 recessive. In this character, then, dominance almost always fails to show 

 itself in the heterozygote and often fails in pure dominants. 



The series of diminishing potency has now brought us to a point where 

 we can interpret a case of great difficulty, namely, a case of rumplessness. 

 Here a dominant condition was originally mistaken for a recessive condi- 



8 



