288 MR. JULIAN S. HUXLEY ON 
of stimulative display characters. If the barbarism be permitted, the terms 
" heterospecitic " and " autospecific " psychical selection might be employed 
to make this distinction. But once epigamic characters come to be 
advantageous, the mind of the species (in the females in sexually dimorphic 
forms, in all individuals in those with mutual courtship) is exerting the 
indirect effect we have been describing upon the future development of 
colour, structure, and behaviour in the race. This is the most important 
fact which Darwin perceived, and this stands firmer than ever in spite of 
the rejection of the bulk of the other part of his doctrine. 
In concluding, I should like to thank Professor Lloyd Morgan, Mr. Eliot 
Howard, and Mr. A. M. Carr-Saunders, all of whom have kindly read the 
foregoing article in manuscript, and have helped me with several important 
criticisms and suggestions. 
Naw College, Oxford, 
March, 1923. 
Postscript 1. — Since the above was written. Dr. J. C. Mottram has been 
good enough to write to me on a number of points concerning sexual 
coloration in birds, and to allow me to see the MS. of an unpublished paper 
on the subject. I would like to take the opportunity of dealing with a few 
of the points which he raises in this and in his book f 14). 
He points out the great importance in many birds of concealing 
coloration — a proposition in which every naturalist who has studied 
birds in the field would agree with him. I have in the body of the 
paper dealt with the ways in which the necessity for concealment or its 
absence reacts upon the " courtship " characters and activities of birds. 
With reference to the Kingfisher [Alcedo ispida), Dr. Mottram makes out 
a strong case for believing that its brilliant colouring, present of course in 
both sexes, is aposematic, with the function of warning possible enemies of 
the bird's unpalatability. He points out that records of the Kingfisher being- 
attacked by birds of prey are extremly rare ; and has found that its flesh is 
unpalatable to man and rejected by domestic animals. 
However, if the Kingfisher really, as seems probable, presented an example 
of warning coloration, it would in no way invalidate my general conclusions; 
it would merely corroborate from a new angle what I have been urging in 
this paper — viz., that each species of bird must be worked out on its merits, 
and that. coloration and behaviour are always determined not by one single 
cause, but by several, of which the two most important are (a) the bird's 
relations with its mate, and (b) its relations with its enemies and its prey. 
Dr. Mottram, however, goes farther. He attempts to account for all 
" courtship '' characters and actions solely in terms of the bird's relations 
with enemies and piey. 
(1) By an ingenious train of reasoning he points out that where the hen 
alone, or chiefly, incubates, she must be regarded as biologically the more 
