v COLOURS OF ANIMALS 379 
is, by the supposition that colour and ornament are strictly 
correlated with health, vigour, and general fitness to survive. 
We have shown that there is reason to believe that this is 
the case, and if so, conscious sexual selection becomes as 
unnecessary as it would certainly be ineffective. 
Greater Brilliancy of some Female Birds 
There is one other very curious case of sexual colouring 
among birds—that, namely, in which the female is decidedly 
brighter or more strongly marked than the male, as in the 
fighting quails (Turnix), painted snipe (Rhynchea), two 
species of phalarope (Phalaropus), and the common cassowary 
(Casuarius galeatus). In all these cases it is known that the 
males take charge of and incubate the eggs, while the females 
are almost always larger and more pugnacious. 
In my “Theory of Birds’ Nests” (see p. 132) I imputed 
this difference of colour to the greater need for protection by 
the male bird while incubating, to which Mr. Darwin has 
objected that the difference is not sufficient, and is not always 
so distributed as to be most effective for this purpose; and he 
believes that it is due to reversed sexual selection—that is, to 
the female taking the usual réle of the male, and being chosen 
for her brighter tints. We have already seen reason for 
rejecting this latter theory in every case; and I also admit 
that Mr. Darwin’s criticism is sound, and that my theory of 
protection is, in this case, only partially, if at all, applicable. 
But the theory now advanced, of intensity of colour being 
due to general vital energy, is quite applicable; and the fact 
that the superiority of the female in this respect is quite 
exceptional, and is therefore probably not in any case of very 
ancient date, will account for the difference of colour thus 
produced being always very slight. 
Colowr-development as illustrated by Humming-birds 
Of the mode of action of the general principles of colour- 
development among animals, we have an excellent example in 
the humming-birds. Of all birds these are at once the 
smallest, the most active, and the fullest of vital energy. 
When poised in the air their wings are invisible owing to 
the rapidity of their motion, and when startled they dart 
