268 MR. JULIAN S.- HUXLEY ON 
For the present, however, I prefer to think of these as ceremonies 
actually connected with mating-up. If so, they are interesting, as they 
would be, so far as I know, the first pre-mating ceremonies described which 
were " mutual,^' in that both sexes played similar roles. Further observations 
are urgently needed, not only on this particular point, but on the whole 
problem of pre-mating ceremonies, especially in species with mutual post- 
mating courtships. 
In any case, the post-mating ceremonies are essentially mutual. The 
commonest is the " snake " ceremony, the incentive to which may, to judge 
from mj' observations, be given by either sex. This ceremony, like the 
pre-mating ones, may often be " self-exhausting " in the sense that, as in the 
Crested Grebe, it need by no means always or usually lead to coition, but 
may be followed by a relapse into ordinary non-sexual routine. Here, 
however, I should like to put on record that further observation on this 
species and on Grebes has convinced me that such ceremonies may often 
have a stimulative effect, although that effect may be gradual and in a sense 
cumulative. It is very common in the Grebe to see a pair repeat a series of 
mutual ceremonies at frequent intervals, and finally go off in the direction of 
the nest and pairing-platform ; this has also been observed in the same species 
by Seloas, and clearly occurs to some extent in the Red-throated Diver. 
That in certain circumstances the ceremony may be completely self- 
exhausting is perfectly intelligible : like the song of most Passeres, the 
performance of the ceremonies is definitely pleasurable in certain states of 
emotional tension. Physiologically, it is then performed " for its own sake" ; 
it may or may not, according to various circumstances, have the further 
biolooical function of raising the emotion to a pitch at which coition is 
desired. 
On the other hand, there is no evidence that in the Divers there exist special 
attitudes and ceremonies especially used as a symbol of readiness to pair, as 
in the Grebe. The approach to the pairing-platform is in itself a symbol of 
some deo'ree of readiness to pair. The emotion associated with this may 
apparently again be dissipated by excited actions, such as the pulling of weed 
from the bottom, and by the ascent of the pairing-platform Avithout actual 
coition. 
The situation of the pairing-platform in a mossy situation is of interest. 
Moss is often but apparently by no means always used as nest-material, and 
in any case always in very small quantities ; the birds can never handle it in 
such large amounts during nest-building as they do in the vicinity of the 
pairing-platform. Two hypotheses might be considered in regard to this. 
In the first place, it might be a reminiscence, the birds having in the past 
employed more moss in nest-construction than at present. Or it might 
represent a specialization ; once the divergence between nest and pairing- 
platform had arisen, and the emotions connected with coition had become 
attached exclusively to the latter, new ceremonies, based on the handling 
