290 • ME. JULIAN" S. HUXLEY ON ' 
that because an earthworm gets on without eyes, and yet reacts to liglitand 
darkness, therefore when eyes are found, they can have no relation to light- 
perception ; or that because lower mammals get on with a small brain, 
therefore the difference in brain-size between them and man can hiive no 
relation to the functions carried out by the brain of lower mammals ; and 
so on, ad lib. 
It becomes increasingly clear that to interpret the behaviour and 
evolution of a bird, even if apparently in only one regard, it is necessary to 
take into account a/Hhe circumstances of its life. As regards coloration 
and " courtship " behaviour, I hope I have shown that relation to enemies 
and relation to the mate are two factors of greatest importance. I am 
grateful to Dr. Mottram for having brought to my notice a number of points 
concerning the relation of birds to their enemies of which! was not aware ; 
and I believe that his contribution to the theory of the subject is of real 
value. But so long as cases remain — as thej' do in large numbers — where 
coloration, structure, or behaviour have an obvious function in regard to the 
opposite sex, and no discoverable function in relation to the enemies or any 
other factor of the bird's life, I maintain that we must take these into account, 
and that Dr. Mottram is definitely wrong in attempting to base his theory of 
the evolution of courtship solely on one, instead of on both of the two main 
factors mentioned above. 
On the other hand, his suggestion that the bright colour and general con- 
spicuousness of the male may have in some species the I'unction of drawing 
the attention of enemies from the biologically more valuable female will, 
I believe, prove of importance in certain puzzling cases. I will mention but 
two. In the first place we have the well-known fact that many mimetic 
species of butterflies, including some of the most striking examples of mimicry 
(e. g. Pa2nlio dardamis), are mimetic in the female sex alone^ the male being- 
conspicuous but non-mimetic. The males are "valuable" onl}^ before copu- 
lation : the females until after oviposition, and further, have usually a poorer 
flight. If a certain average toll is taken of the species by enemies, it could 
well be a real advantage to have it fall preponderatingly on the male sex. 
^ The second example concerns birds. Reflection will show that the bright 
colours of male birds fall broadly into two categories — those which, like* the 
" tail " of the Peacock or Argus Pheasant, are only conspicuous in display, 
and those which, like the general coloration of most male ducks, are always 
conspicuous. It will be further found that the former are definitely displayed, 
the latter usually not. -E. f/., the display of male ducks is largely a bobbing 
of the neck ; the general bright body-colour is not made more conspicuous in 
display. The male duck, furthermore, (i.) takes no share in incubation or 
rearing the young, (ii.) usually stays near the hen until the young are hatched, 
(iii.) as soon as he leaves the lien, moults into eclipse (protective) plumage, 
(iv.) The hen is protectively coloured. ' (v.) The species is highly palatable and 
much attacked by Eaptores. 
