COURTSHIP ACTIVITIES IN THE RED-THEOATED DIVER. 279 
o£ nest-material, because the two activities have been so often actually 
connected. But we cannot adopt this comparatively simple explanation for 
the incorporation of wholly non-sexual activities, such as preening, in court- 
ship. So far as my personal experience goes (and so little attention has been 
devoted to the matter that the literature is of scarcely any help), the birds 
which make most use of these non-sexual actions in courtship are those 
which make least use of the peculiar expressional movements of wings, tail, 
and feathers generally, which we saw made up the raw material of courtship 
actions in most species. If this should turn out to be generally true, then 
we should have to suppose that the emotional tension, since for some reason 
it does not in these birds express itself in the originally non-significant 
methods of general wing, tail, and feather movements, must find an outlet 
elsewhere, in the guise of actions which are already performed by the species 
in other connections. In any case, it is clear that these actions are utilized, 
like the general non-significant actions, as physical relief or expression for 
emotional tension ; and that since they are already given in the bird's 
instincts and habits, they form the natural raw material — the natural outlet 
for surplus psycho-neural excitement due to emotion temporarily unable to 
find its normal expression. 
In a sense, therefore, the non-sexual associated actions are mere expressions 
of emotion, comparable to the meaningless acts often performed by men 
under the influence of great emotional tension. But once they have come to 
be associated with sexual emotion, they then become the raw material which 
can be worked up into more elaborate courtship ceremonies. Suet), for 
instance, appears to be the origin of the head-shaking ceremonies in the 
Crested Grebe. In diving birds generally, there is an almost universal habit 
of shaking the head on emerging after a dive. This is, as we have seen, 
slightly associated with sexual emotion in the Diver, but in the Grebe the 
head-shaking is a veiy elaborate part of highly-specialized ceremonies, and 
its origin from ordinary non-sexual head-shaking can only be deduced from 
indirect evidence. Unless, however, its origin be of this nature, it remains 
quite obscure. 
The association of diving with courtship in both Grebe and Diver may be 
adduced as further proof of our contention. The special " splash-dives '^ in 
the Diver, and the emergence from the water in a vertical attitude which 
occurs in both species, are both obviously modifications of the ordinary 
diving practised for food, which have now an exclusively emotional 
significance) and a sexual function. Under the influence of unsatisfied 
sexual emotion the bird indulges in various forms of action habitual with 
it ; later, specialization has taken place so that the sexual modes of diving- 
become quite different in appearance from the original non-sexual mode. 
As a matter of fact, when we survey the varieties of courtship, we find 
that they are not uncommonly to be regarded as specializations of normal 
