284 MR. JULIAN S. HUXLEY ON 
Recently investigations have been made on breeds o£ fowls like tlie Sebright 
Bantams, in which the cock is heu-£eathered. Two interesting points have 
emerged. The first is that the condition is due genetically to a Mendelian 
gene, the second that it is due physiologically to the fact that the male gonad 
in these animals possesses the same substance as do the gonads of normal 
females, which inhibits the development of male plumage. This latter point 
is demonstrated by castrating hen-feathered cocks, upon which they become 
cock-feathered — a strange but conclusive result (Morgan, '19). Morgan 
believes that the condition has its histological basis in the development of 
luteal cells in the testes of hen-feathered cocks, similar to those found in the 
ovaries of normal hens, but recently Pease ('19 ; </. v. for references to previous 
work) has thrown doubt on this. 
• In any case, the facts are of great interest as showing by how simple a 
means, and in how few generations, a dimorphism of plumage could be 
converted into a similarity. On the other hand, it also shows that in this 
dimorphic species at least, plumage and instincts (as the castration experi- 
ments also demonstrate) depend upon different mechanisms for their develop- 
ment. The development of male or female instincts is due to a positive effect 
exerted by some secretion of the male or female gonads respectively, acting 
upon a basis which is neutral in the absence of the specific secretion, wliereas 
the male gonad has no positive effect upon' plumage development (the 
plumage of capons being, if anything, more male than normal), while the 
female gonad exerts in this respect an entirely inhibitory effect. 
It must be emphasized, however, that we have no experimental information 
with regard to species with similar plumage and courtship (in passing, it 
may be remarked that the castration of Grebes or Herons should provide 
interesting results). Further, from the general considerations advanced 
above, it is clear that we should not expect precisely the same mechanism to 
hold good in "mutual" as in dimorphic species. In the latter, as we have 
seen, selection must have been at work emphasizing and exaggerating any 
sexual differences of instinct which primitively existed ; whereas in the 
former all the influence would tend in the other direction, of assimilating 
instinct in the two sexes. 
To what pitch this has as a matter of fact been carried is shown by the 
observations of Selous and myself upon the Crested and the Little Grebe 
(Dabchick). The Grebes are birds in which mutual courtship attains the 
highest development yet described, and in which sexual adornments similar 
in the two sexes attain a great specialization. In the two species mentioned, 
and probably in others, the instincts of the sexes are so alike that coition 
occurs both in the normal or in the reversed position. In these birds, coition 
always takes place upon a nest; the "passive" bird extends itself at full 
length in a perfectly horizontal position, the "active" bird mounts the other 
from behind, and, after coition, walks up its body and off by its shoulder. 
