278 ME. JULIAN S. HUXLEY ON 
elongated and dissected^ o£ Egrets, together with some of the peculiarly 
situated plumes o£ Humming-birds and Birds-of-Paradise. 
It appears, then, that "courtship" consisted at first chiefly of non- 
significant movements employed by the male as an outlet for the pent-up 
sexual emotion, these being in part derived from those employed in coition, 
and that special colours and structures displayed only mainly during 
these movements are later specializations. If so, then we should expect 
to find some species of birds in which the movements exist, but not the 
special colours or structures — " courtship " actions without epigiimic colours 
or structures. This is as a matter of fact the case. The Sylviidse, for 
instance, give us many examples of species which are not dimorphic in 
colour, or only so to an obviously non-significant extent, but are dimorphic 
in behaviour, usually the males alone performing special post-mating 
ceremonies, apparently stimulative in function, in which wings, tail, and 
feathers are moved in the general way that has been described 
(E. Howard, '20). Rarely, however, both birds perform ceremonies simul- 
taneously, as is sliown in fig. 8 (PI. 15). But the two perform ordinary uni- 
lateral ceremonies, and not in any common rhythm. There is no sign of a 
unified mutual ceremony. It is an interesting intermediate type of behaviour. 
Usually, however, the close observation of the courtship of a species 
reveals other behaviour which cannot be brought under the rubric of 
originally non-significant physical release of emotional tension. I refer to 
actions which are to be thought of as having originally been developed in 
connection with certain other functions, and , later having been connected 
with sex in courtship displays. Such actions have already been referred 
to. Sometimes they are to a certain extent associated with sexual emotion, 
as are all those connected with nesting-material ; sometimes they appear 
to have no such connection, such as the preening, head-shaking, dipping 
of the bill, &c., which we have noticed in Divers and Grebes. In Mute 
Swans, I recently observed some interesting courtship-actions, and there 
one of the prominent pieces of behaviour was the dipping of the whole 
head and fore-part of the bod}'' below the water ; the action was strongly 
reminiscent of an incipient attempt at reaching down to feed in the 
characteristic swan manner. The birds also repeatedly preened themselves in a 
manner very similar to that seen in the Grrebes — i. e. a manner indicative of 
the action being used in what I may call a ritual way, and without any 
of its usual functional significiince. I am confident that when once attention 
has been drawn to this " ritual '■" use of non-sexual actions during courtship 
activities, it will be found to be of very wide occurrence. Prof. H. Balfour 
informs me that it occurs commonly in the Gannet. The association of 
nesting-material with courtship is certainly very widespread, and here we 
may probably see the working of direct association. The sexual emotion 
directly activates the centres connected with nctst-building and the handling 
