GENERAL DISCUSSION. 87 



ceases. The growing character is a youthful, embryonic one; the new 

 character is the stoppage of growth. Similarly the young feathers of birds 

 grow continuously until something intervenes that stops the growth and 

 dries up the sheath. Now, in Angora rabbits and long-tailed fowl the 

 epidermal organ continues its embryonic growth indefinitely; the some- 

 thing that intervenes to stop growth is absent. There is no reason for 

 regarding the long hair or long feather as a positive condition and short 

 hair or feather as due to its absence. 



Again, Mediterranean fowl have non-feathered shanks; but in Asiatics 

 the feet are feathered Hke the rest of the body (except the soles and face). 

 It has been assumed that 600^ is an additional character and should be 

 dominant over absence of boot. But, on the other hand, we may well 

 think of the capacity of producing feathers as general to the skin. From 

 this point of view the real question is, what prevents feather production 

 on the eyelids, comb, wattles, and shank? It seems equally probable that 

 there is an inhibitor of feather-growth for these few areas as that every 

 conceivable area of the body has its special stimulus factor for feather 

 development; or even as that there is such a factor to each separate feather- 

 tract. In the Minorca, then, the inhibitor of boot is present; in the Silkie 

 a weak heterozygous inhibition appears; but in the Dark Brahma there is 

 no inhibitor and feathers extend down from the heel over the whole of 

 front and sides of the foot and even on the upper surface of the toes — just 

 as they do over the anterior appendages. 



The case of the rumpless fowl is important in relation to the hypothesis 

 of inhibitors. Either tail-production depends on a special factor TT, which 

 is diluted, as Tt, in the heterozygote; or else there is a tail inhibitor, //, which 

 is diluted, as li, in the heteroz3'gote. In Fj we expect, on the one hypothesis, 

 25 per cent tt, giving no tail, and 25 per cent TT, giving tail; on the other 

 hypothesis 25 per cent ii, giving tail, and 25 per cent //, giving no tail. Actu- 

 ally we get all tailed in some cases ; in others 25 per cent with no tail. Which 

 hypothesis best fits the facts? Which is the more probable — that the 25 per 

 cent recessive no-tail should produce a tail (as it were, out of nothing) or 

 that the 25 per cent dominant tail inhibitor should be ineffective, permitting 

 the development of a tail? It is clear that the ontogenetic failure of an 

 inhibitor is easier to understand than the development of a character that 

 is not represented at all in the germ-plasm. This matter is treated in 

 another connection in the next section. But the present point is that it is 

 equally in accord with the facts to regard heredity as initiating and inhibit- 

 ing processes. If, indeed, processes were not regularly inhibited, they must, 

 when once started, go on indefinitely, as do the hairs of Angora goats and 

 wonder-horses. 



As we have seen, ontogeny is not completed at hatching or birth. 

 Many characters are at that time undeveloped. Hence, not infrequently 

 the recessive condition is at first seen and is only later replaced by the 



