86 INHERITANCE OF CHARACTERISTICS IN DOMESTIC FOWL. 



We have reason for concluding that each developmental process is a 

 "response" — a reaction of the living, streaming protoplasm to changing 

 enviroimient. The nature of the response to any stimulus probably de- 

 pends on the chemical constitution of the protoplasm — and this is hereditary. 

 In an important sense heredity is the control of ontogeny. 



The specific characteristics are mostly those that appear late in ontog- 

 eny. The integumentary folds over the nasal bones of the chick appear 

 on or about the tenth day. At that time it can be ascertained whether the 

 comb is median, or multiple, or Y-shaped, or cup-shaped, or consists of 2 

 papillae. In the case of the single-comb the fold is Unear and single; in the 

 case of the pea-comb, Unear and triple; in the case of the rose-comb, quin- 

 tuple or irregularly wrinkled over the whole area; in the case of the Polish- 

 comb, there is a pair of "pocket folds." In the single-combed fowl the 

 single Hnear fold grows quickly to a great height and very thin, while in 

 the pea-comb, with its additional pair of wrinkles, the median element is 

 not so high as in typical single-combed races; in the pea-comb there is an 

 additional folding stimulus and a reduced growth stimulus. In the hetero- 

 zygote both stimuli are weakened; the lateral folds are usually much 

 reduced — " are hard to make out, " as I stated in 1906 (p, 35) ; and the factor 

 that determines the continued growth (elevation) of the fold is weakened, so 

 that the pea-comb — although "abnormally high" (1906, p, 35, figs, 20 and 

 21) — ^is not nearly as high as the single-comb of the Minorca (1906, fig, 4). 



Two results are evident: first, each character in the heterozygous 

 condition is reduced, and, second, each is much more variable than in the 

 homozygous condition. Why is the character reduced? If the reaction to 

 continued growth of the fold is strong in one race and weak in the other, 

 then in the heterozygote that reaction, whatever its nature, is reduced. 

 Why is the reduction in the response so variable? There is a variation in 

 the irritability or other growing factor of the embryonic material that is 

 destined to form the fold. Even Minorcas vary in the growth of the comb, 

 and so do the Dark Brahmas. Let (r be a constant element of the growth 

 factor of the Minorca's comb; then G + a or G — a will indicate its variants. 

 Let g be the growth factor of the Brahma's comb, and g + a and g— aita 

 variants. Then the hybrids of these two races may be of the following types : 

 Gg, Gg + a, Gg- a, Gg + 2a, Gg - 2a. This gives 5 varying conditions 

 instead of 3 and greater extremes of variation. 



In the foregoing case I have assumed that the positive character is 

 that of increased growth in the Minorca; but the positive character may 

 be an inhibition to indefinite growth of the pea-comb. Heredity may 

 be conceived of as exerting at all points a control on developmental pro- 

 cesses — sometimes initiating and continuing this; but often, on the other 

 hand, slowing down or wholly inhibiting that. The inhibition of a process 

 is quite as positive a function of heredity as its initiation. The hair of a 

 young rabbit grows until it attains a certain length and then the growth 



