The _specific_ characteristics are mostly those that appear late in ontogeny. 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 papillæ. In the case of the single-comb the fold is linear and single; in the case of the pea-comb, linear and triple; in the case of the rose-comb, quintuple 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 linear 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 heterozygote 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 _G_ 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 - a_ its 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.