90 INHEEITANCE OF CHARACTERISTICS IN DOMESTIC FOWL. 



statements that the progressive condition and the "present" factor are 

 dominant. 



The definition of dominance on the ground of results meets at the 

 outset with a difficulty the germ of which is observable in Mendel's cautious 

 statement "ganz oder fast unverandert. " Even Mendel observed that the 

 hybrids between white-flowered and purple-red flowered peas have flowers 

 less intensely colored than the darker parent. The experiments of the last 

 seven years have shown that the "dominant" character is often very greatly 

 changed — indeed, in extreme cases a blending of characters may occur — 

 in the first generation. Correns (1900 6, p. 110) very early stated that in 

 a certain set of crosses between good species the hybrids showed the char- 

 acter of both parents, only reduced, but in varying degrees. Bateson and 

 Saunders (1902, p. 23) found in crossing two forms of Datura that — 



Although the offspring resulting from a cross between any two of the forms 

 employed are usually indistinguishable from the type which is dominant as regards the 

 particular character crossed, yet in other cases the intensity of a dominant character 

 may be more or less diminished either in particular individuals or in particular parts of 

 one individual. In Tatula^ Stramonium cross-breds the corolla is often paler in color than 

 that of the dominant parent (as has already been noticed by Naudin), but even in the 

 palest specimens the deep blue color of the unopened anthers leaves no doubt as to the 

 presence of the dominant color element. * * * The occurrence of intermediate 

 forms was also occasionally noticeable in the fruits. Among the large number of capsules 

 examined, there were some of the mosaic type, in which part of the capsule was prickly 

 and the remainder smooth, while others, suggesting a blend, were more or less prickly 

 all over, but the prickles were much reduced in size, and often formed mere tubercles. 



Bateson and Saunders further showed (1902, p. 123) that in the case 

 of comb and extra-toe in poultry " the cross-bred may show some blending 

 and * * * the intensity of the dominant character is often consider- 

 ably reduced." 



Correns (1905, p. 9) pointed out that there was known, even at that 

 time, a complete series of cases at one extreme of which one determiner 

 completely hindered the appearance of the other, while at the opposite end 

 of the series the hybrid showed an intermediate condition, both determiners 

 appearing with equal strength. 



The following year, in my first report on Inheritance in Poultry, I laid 

 great stress on the imperfection of dominance, and this phenomenon has 

 become more striking and clear in the subsequent years, until in the present 

 paper it is recognized as the key to the explanation of many apparently 

 anomalous types of heredity. 



The first case in the present work in which imperfection of dominance 

 is considered is that of the hybrids between I and oo comb. Here median 

 comb is mated with no-median. Each somatic cell of the hybrid— at least 

 in the comb region— has only half the full determiner for median comb. 

 The determiner is weakened, and so the median comb is imperfectly devel- 

 oped, namely, at the anterior end of its proper territory. The weakening 



