282 MR. JULIAN S. HUXLEY ON 
grounds are so distant; furthermore, protective coloration, as in most 
colony-breeding birds, is unnecessary. Here we find two tendencies acting 
in the direction of similarity in appearance and behaviour. In the first 
place, there is no need for the female to be inconspicuous, whether in 
plumage, voice, or action ; thus, ceteris paribus, there will be no tendency to 
restrict any variations in the direction of greater conspicuousness — even if 
they may have been first acquired by the male sex, which is by no means 
necessary — to the males. Secondly, since the male as well as the female 
must possess the instinct to incubate and to feed the young, it follows that 
there is a tendency to approximate the instincts of the two sexes in these 
particulars ; and the sex-similarity thus initiated again tends to be con- 
tinued in other ways, tinloss there are reasons to the contrary, which, in birds 
to whom inconspicuousness is a matter of indifference, we have just seen do 
not exist. In birds of this type, therefore, there exist again two mutually 
reinforcing tendencies which tend to bring about a similarity of plumage 
and instincts, and so lead to mutual courtship. 
There are, it is true, species like certain Penguins (Levick, '14) in which 
the sex;es are alike, but much of the courtship is not mutual ; there are other 
species, like our Red-throated Diver, in which the sexes are similar and 
there exists mutual courtship, but in which the male helps to a comparatively 
small extent in incubation. In such a complicated subject we should not 
expect to find universal hard-and-fast rules ; each case requires to be 
investigated on its merits, and with a thoroughness which has so far only 
characterised few workers. But that the two general tendencies which 
I have mentioned, to dimorphism and to mutualism, do exist, and that they 
are definitely correlated with other characteristics of the species' life-history 
in the general way I have sketched, appears to be incontestable. 
One other point remains to be considered here. Why is it that among 
birds with mutual courtship, some, like Divers and Grebes, perform cere- 
monies far from the nest, no emotional expression being seen at or close 
to the nest during incubation and the care of the young, whereas in others, 
such as Herons, Egrets, Pelicans, &c., the chief ceremonies observed are 
those which take place at the nest or nest-site, especially at the moment when 
one bird relieves the other from the duty of incubation ? (PI. 14. figs. 5 & 6). 
The answer is not, I think, far to seek. The " nest-relief birds," as we 
may call the latter category, are all species in which the nest is not concealed, 
and does not appear to be in need of special protection against enemies. In 
most cases the birds nest in colonies. The birds with mutual ceremonies 
which are unconnected with nest- relief, on the other hand, have nests which 
are carefully concealed, or at least are far from conspicuous, and rarely breed 
in colonies. 
When a sitting bird is rejoined by its mate, especially if the mate's 
presence implies a speedy relief from the tedious business of incubation, 
