276 MR. JULIAN S. HUXLEY ON 
Harper's observations (discussed by Marshall, '22, pp. 1.34-135) make it clear 
that the female pigeon will not ovulate unless stimulated emotionally. This 
stimulus is normally provided by the male ; but two females isolated together 
will often lay. Single females when isolated never lay (except occasionally 
when overfed, and then only a few eggs). The recent work of Chance {'22) 
indicates that in the common Cuckoo ovulation occurs under nervous control, 
the sight of a pair of foster-parents building their nest acting as a stimulus. 
It is obvious, further, that the willingness of female birds for coition is .not 
almost exclusively a matter of internal secretion as in mammals, but is largely 
under emotional control. It is also well known to field observers that the 
state of the weather has a marked effect upon the emotions and their 
expression in birds ; song and display is regularly far poorer on a cold, dull 
day than on a bright, warm one. Other things being equal, therefore, 
anything which tends to counteract the depressing effect of bad-weather 
conditions upon the emotions and the actions under their control vvill be of 
biological advantage in that the number of ovulations will be increased, and 
also the number of coitions, and so the chances of fertilization for the eggs 
after ovulation. There would thus in ordinary seasons be no marked 
biological advantage gained from stimulative ceremonies ; this would accrue 
in exceptionally bad seasons only. 
It appears, as I have already indiciiteJ, that in birds with mutual courtship 
the ceremonies do have a stimulative effect, although this is usually of a 
general and indirect kind, keeping the level of sexual emotion up to a con- 
stant pitch, and occasionally heightening it so that visits are made to the 
pairing-platform, &c., where coition will usually occur. It is highly probable 
that a similar general stimulation is exerted by the more primitive displays 
of the "territory" birds, although here the stimulation is not mutual but 
unilateral. 
It is further clear that any display which did have such an effect would 
reap the biological advantages, mentioned in the preceding paragraph, of 
counteracting the bad effects of cold seasons. As a matter of fact, even in 
these bad seasons some nests will be found with full clutches of eggs and 
normal proportion of fertile eggs. There is, therefore, a form of natural 
selection actuallj^ in progress here, which (as often) is only operative in 
particularly adverse conditions. 
It is of very great importance that this point should be cleared up, and 
that field ornithologists should undertake careful statistical work on the 
relation between weather, number of eggs in a clutch, and percentage of 
infertility. It is only so that firm bases can be found for biological theories. 
It must now be asked, as regards stimulative displays, what form of 
stimulus has been adopted and why. So far as males of dimorphic species 
are concerned, what is the position? It is that they are subjected to a 
mental state which is comparatively rare in organisms below man, of being- 
stimulated by a powerful emotion but being unable, so long as the female 
