286 MK. JULIAN S. HUXLEY ON 
then there will result, unless special circumstances arise, a simultaneous 
dimorphism in both appearance and instincts ; if they remain on the whole 
similar, there will be (again unless there are special reasons against it) a 
similarity" between the sexes, not only as regards appearance, but also as 
regards courtship and parental behaviour. 
If this turns out to be well founded, and the gonads do normally in birds 
determine appearance, behaviour, and size simultaneously, then it is clear that 
observation will have contributed important guiding lines for future genetic 
and evolutionary research. 
In conclusion, I would like to emphasize the fact that, even when both sexes 
are similar, there is often no doubt whatever that the coloration and special 
structures used by them both in courtship have been evolved in relation to 
sexual habits, and in essentially the same wa)' as bright coloi;rs and special 
structures used in courtship by males alone in dimorphic species. They have 
arisen through the necessity of providing an emotional stimulus to the other 
member of the pair, if fertilization is to be normally and easily effected, 
particularly in the face of adverse environmental conditions; and from the 
reverse point of view, have become necessary because it is biologically 
desirable to keep the mean sexual excitability of the pair at a level low 
enough to prevent excessive coition. They are, to use the useful term of 
Poulton's which I have for some time adopted, always epigamic, although not 
always secondary sexual characters. They have also been usually grafted on 
to primitive ceremonies which are the direct outcome of sexual excitement, 
and indirectly often the result of an "engagement'' period which is caused 
by non-sexual exigencies of the life-cycle. 
Although the direct competition among males, which was one of the main 
points of Darwin's theory of sexual selection, does not appear to occur in the 
evolution of most epigamic characters among birds, yet it is noteworthy that 
in the polygamous species in which it does occur, the type of coloration and 
structure which is evolved is very similar to that evolved in monogamous 
species, except that development has usually gone to further lengths of 
specialization. This similarity between the end-results in what we may call 
" accessory '■' as well as in " sexually-selected " epigamic characters, is due to 
the second salient fact first clearly recognized by Darwin — the fact that the 
development of an epigamic character is dependent upon the emotional effect 
which it produces upon the mind of a bird of opposite sex. Epigamic 
characters must be emotionally stimulative if they are to perform their 
function successfully ; and to be emotionally stimulative, it would appear 
that they must be first striking, and secondly not usually yisible, so as to have, 
when they are displayed, an element of novelty and uufamiliarity which adds 
to their stimulative etfect. So far as their striking quality is concerned, they 
may be either what seems to us beautiful or else what seems to us bizarre. 
So far as unfamiliaiuty goes, it should be remembered that many epigamic 
