COURTSHIP ACTIVITIES IN THE RED-THROATED DIVER. 283 
it is natural that emotion should be aroused. If there is no biological reason 
against the expression oli the emotioUj it will naturally be expressed then and 
there in some form of ceremony. On the other hand, if there are biological 
reasons for concealing the position of the nest from enemies, then it will be 
desirable not to draw attention to the spot by conspicuous sounds and actions, 
and selection will see to it that the emotion is not expressed immediately 
and natui-ally at the nest, but at a distance from it. The fact that, in the 
Crested Grebe, there exists a special and very elaborate ceremony which 
occurs when the members of the pair rejoin each other after having been 
separated for some time, indicates that in this species too the same emotions 
occur in similar circumstances, but are not allowed expression in the neigh- 
bourhood of the nest. 
Here again, therefore, we find an influence of the general mode of life upon 
the form of the courtship. So that we may really say that, although, given 
in the first place the state of general excitement produced by the sexual 
hormones, in the second place the stimulative effect of sexual display, and 
thirdly the necessity for regulating coition, there will inevitably come into 
being displays, &c., which go by the general name of courtship, yet the form 
of the courtship, not merely in details but in broad lines as well, will depend 
in the main upon other general biological factors affecting the species. 
There remains to be considered the physiological and genetic mechanism 
which we must suppose responsible for the phenomena ; but before passing 
to this, I should like to summarize briefly the conclusions just reached. In 
few words they are these : — 
1. Polygamy encourages sexual dimorphism. It is only found among 
birds with nidifugous young, since nidicolous young require that both parents 
should assist in leeding. 
2. Need for inconspicuousness encourages dimorphism. 
3. Nidifugous habit in the young facilitates tlie development of dimorphism 
otherwise initiated. 
4. Early occupation of territory by one sex encourages dimorphism. 
5. Absence of need for inconspicuousness encourages mutualism. 
6. Need for incubation by both parents encourages mutualism. 
In addition, there are no doubt many special physiological points which 
encourage one or the other tendency ; but in general the condition observed 
is the result of the interaction of the tendencies we have enumerated. 
Recent experimental work is beginning to give us an insight^ albeit but a 
partial one as yet, into the physiological and genetical mechanism underlying 
sex-characters in birds. 
It appears that castration of a male, in highly dimorphic species like fowl 
and wild duck, produces birds which are neutral in instincts but remain male 
in plumage. 
In females, on the other hand, castration produces cock-feathering, but not 
male instincts. 
