524 MR. J. S. HUXLEY ON THE > 
some other process? Such other process can easily be imagined, 
and I feel confident that it has played a considerable part. We 
may call it Double or, better, Mutual Sexual Selection (Mutual 
Selection for short). Where combined courtship-actions exist, 
and a variation in the direction of bright colour or strange 
structure occurred, it would make the actions more exciting and 
enjoyable, and those birds which showed the new variation best 
would pair up first and peg out their “territories” for nesting 
before the others could get mates. The level would tend to be 
raised generation by generation. Mutual Selection is in a way 
a blend between Sexual and Natural Selection. The structures 
and actions arising under it have their immediate origin in the 
preferences of individual birds, not in anything outside the 
species, and in their immediate function they are entirely confined 
to the courtship. On the other hand, the mutual courtship 
itself, the activities of both birds taken together, may be of use 
to the species as a whole, in keeping the sexes together when 
necessary. ‘Then the indirect function of all the shaking-bouts 
and displays of the Grebe is a function of use to the species, and 
besides the direct origin there is added an indirect origin under 
the pressure of Natural Selection. 
Mutual Selection has a certain similarity with assortative 
mating, but is by no means the same thing. Like true Sexual 
Selection, it encourages an ever higher level in the development 
of a character, once variation has given it a basis to start from. 
In the Grebe the line of variation encouraged by Mutual Selection 
has been the tendency to produce ruffs and tufts of feathers on 
the head, and to go through actions involving, besides the use of 
these structures, diving and sporting with water-weed. 
The question in the Grebe is complicated, as noted above, by 
the slightly less developed crest of the hen ; this, however, might 
easily be accounted for by differences in the metabolism of cock 
and hen. The Discovery and (especially) the Display Ceremonies 
are also rather stumbling-blocks in the way of an explanation by 
Mutual Selection ; they seem so very like the Displays of solitary 
courtship. However, even here the second bird plays a part, 
which in the Discovery Ceremony is at least as important as that 
of the displaying birds. 
What is quite clear, however, is that, even supposing (what 
to me personally appears very doubtful) that ordinary Sexual 
Selection has “produced” the structures and the cat-position 
(we must know more about the habits of other species of the 
genus to decide this), yet it has gone hand-in-hand with a 
process of Mutual Sexual Selection as regards the majority of the 
actions. These actions (like the display of the Peacock, but 
unlike that of the Warblers) are much too elaborate and much 
too specialized to be considered as the immediate outcome of any 
form of physiological excitement. They obviously have a long 
and complicated evolution behind them, and, as they can only 
