88 INHERITANCE OF CHARACTERISTICS IN DOMESTIC FOWL. 



dominant condition. The reverse sequence will rarely be followed, because 

 development rarely, except in cases of degeneration, moves backward. 

 One of the famiUar cases of this sort is human hair-color. In youth this is 

 frequently flaxen, later it becomes light brown, and eventually it may 

 become dark brown. Darwin gives a number of examples in his Chapter 

 XII of Animals and Plants under Domestication. To these I may add 

 some from my own experience. The hybrids between white and gray Java 

 sparrows are at first Ught and later become of a slaty gray like the dark 

 parent. Many black fowl gain white feathers as they grow older, and every 

 fancier knows that birds with complex white-and-black patterns can usually 

 be "exhibited" only once, on account of loss of "standard" coloration 

 late in life. In these cases the advanced condition in the series of melanic 

 colors appears only late in ontogeny.* Similarly Lang (1908, p. 54) finds 

 that in snail hybrids often the young shells have the recessive yellow color, 

 only later in life showing the dominant red color. This is, of course, no 

 reversal of dominance in ontogeny, but mere ontogenesis of pigmentation. 

 So in general, since the recessive condition is absence of the character or 

 its low stage of development and the dominant condition is presence of 

 the full character, the individual in ontogenesis may exhibit in succession 

 the recessive and then the dominant character, but not in the reverse order. 



B. DOMINANCE AND RECESSIVENESS. 



If segregation is the cornerstone of modern studies in heredity, domi- 

 nance forms an important part, at least, of the foundation. In any case, 

 a critical examination of dominance is now required ; the more so since its 

 significance and value have often been doubted. 



First, how is a dominant character to be defined? It has been defined 

 both on the basis of visible results in mating and on the basis of its essential 

 nature. On the basis of visible results in hybridizing dominant characters 

 may be defined as Mendel (1866, p. 11) defined them: "jene Merkmale, 

 welche ganz oder fast unverandert in die Hybride-Verbindung iibergehen." 

 Bateson's translation (1902, p. 49) renders this passage: "those characters 

 which are transmitted entire, or almost unchanged in the hybridization." 



On the basis of the essential nature of the dominant character there has 

 obtained a great diversity of definitions. Thus de Vries (1900, p. 85) sug- 

 gested that the "systematically higher" character is the dominating one, 

 and, again (1902, pp. 33, 145), that the dominant character is the phylo- 

 genetically older one. Many have suggested that it is the positive or present 

 character that dominates over the negative, latent or absent. This last 

 idea has become the prevailing one and its history is worth summarizing. 



As early as 1902, Correns used as Mendelian pairs, presence of coloring 

 material and absence; also modification into yellow and no modification. 



* Does the graying of human hair represent au ontogenetically advanced condition of the melanic pigment as yellow 

 represents the embryonic condition ? 



