272 MK. JULIAN S. HUXLEY ON 
keyed so high that an undue amount of coition resulted. Further, when the 
level o£ readiness to pair is reached, it does not follow that the bird will be 
under that amount of emotional tension which results in some physical 
expression being necessary : in fact, this appears to be the accompaniment 
of a definitely higher level of excitement. " Courtship " ceremonies which 
result in these circustances may therefore have one of two functions. In the 
first place, they may be performed by one alone ; they may then convey to 
the other that the first is ready to pair — in other words be informative in 
function — or in addition be stimulative, and help to key the other up to the 
requisite pitch. The stretching-out in the passive pairing attitude which 
occurs on open water in Crested Grebes (Huxley, '14) and on the nest in 
Dabchicks (Selous, '15) appears to be primarily such an informative 
symbol, although it is at least probable that it is also stimulative. On the 
other hand, there are many other courtship actions, such as the majority at 
least of the mutual post-mating ceremonies described both for the Diver and 
the Grrebe, which appear to be primarily, or in any case, primitively stimu- 
lative in function. They are not symbolic, in the sense that they are not 
representations of coition attitudes, and both birds take part in them, usually 
though not always performing similar roles. These have as their original 
function the raising of the level of sexual emotion — only here it is a mutual 
raising of an originally similar level. It is true thatj as in the Grebe, these 
ceremonies are frequently what I have called self-exhausting, and do not 
lead to coition, but this is probably secondary ; it is also probable that 
they may have other functions besides that of stimulating to coition. 
Both these points, however, we must discuss later, as also the i-eason for 
the existence of the two different methods of dimorphic and of mutual 
courtship. 
It is no doubt often difE.cult to distinguish between the two ; but in the 
Crested Grebe at least there appears to be a real distinction. Objectively 
(" behaviouristically "), e. g. the pairing-attitude is adopted only near the nest 
or pairing-platform, and is almost always followed closely by coition ; it is 
not accompanied by any of the usual signs of violent nervous activity, nor by 
anything that can be called display. Per contra, the mutual ceremonies may 
take place anywhere, may or may not be followed by coition, and, if so, 
generally after some interval ; they are associated with various signs of 
general nervous activity, and do serve to display special structures and 
colours. 
Psychologically, it would appear that the coition-attitude is narrowly and 
specifically associated with coition, and is " informative," denoting readiness 
to perform the act itself ; while the mutual display ceremonies are not 
associated with coition in particular, but w'itli general emotional excitement. 
In so far, therefore, as they have a stimulative function, it is concerned with 
raising the general emotional level to the pitch required to make the birds 
repair to the neighbourhood of the nest — i. e. to bringing about the general 
