COCRTSHIP ACTIVITIES IN THE RED-THKOATED DIVER. 271 
determining behaviour, and that a stimulation of the emotional centres is a 
necessary preliminary to coition. In a precisely similar way, at an earlier 
stage oi: evolution, a stage of general bodily organization was reached at 
which internal fertlization was desirable or necessai-y- On this stage being 
reached, it became imperative that coition should occur, and that, in practi- 
cally all cases (the majority of birds constitute an exception), specialized 
copulatory organs differing in the two sexes should be developed. Both (a) 
copuliitory organs and (6) most of the courtship ceremonies and epigamic 
structures we have been considering can therefore be classed under the same 
head — viz. as accessory sexual organs. I have heard this point epigram- 
matically snmmed up in discussion by the remark that this type of male 
courtship is a " psychological penis." lis any event, it is clear that jnst as 
no one has ever considered it necessary to postulate a special type of selection 
to account for the origin of copulatory organs, so the origin of courtship, so 
far as it is of this stimulative type, does not demand the special evolutionary 
mechanism of Darwinian sexual selection to explain it. 
The question may now be i-aised as to the relationship between this 
" stimulative " courtship in species with sexual dimorphism and courtship in 
species where the appearance and behaviour of the sexes are similar. To 
answer this we must go back for a moment and ask a more fundamental 
question : — What is the reason, in the dimorphic species, for the difference in 
sexual excitability between male and female ? There may be some primary 
difference between the sexes which tends in this direction, but there can be 
little doubt that the marked accentuation of this, which is seen in most 
dimorphic species among the higher animals, has for one main function, as 
has been several times suggested {e. g. especially by Groos,'98, and E. Howard', 
'07-'14), the regulation of coition. If both sexes were highly excitable, 
coition would occur too often ; on the other hand, it would be undesirable to 
reduce the strength of the Impulse too far, until it was near the lower level 
at which it ran the risk of being extinguished under the pressure of accidental 
unfavourable circumstances. (There is much evidence to show that, e. g., cold 
and damp have a markedly depressing influence on the sexual activity of birds.) 
An obvious method of ensuring the golden mean between the danger of 
excess and that of possible suppression of the instinct is to have one sex with 
a very strong instinct, or at least one which is readily excitable, while that 
of the other requires considerable stimulation to arouse it. (We have already 
seen that the instinct must be aroused through stimulation of the higher 
centres.) 
There is, however, another method of ensuring a similar result, and that is 
to have the sexes at the same general level of excitability, the male less 
excitable than in dimorphic forms, the female more so. In this case both 
sexes will constantly be reaching tlie level of stimulation at which desire for 
coition occurs. On the other hand, here again it is unlikely that both 
will at the same moment spontaneously reach this level — unless both were 
