COURTSHIP ACTIVITIES IN THE RED-THROATED DIVER. 281 
these must possess different instincts from the females — instincts vi^hich 
begin to react to seasonal influences some time before any reaction occurs in 
the other sex. Here we have a primary basis for the rest of the dimorphism 
observed — a basis given by territory requirements, not by epigamic needs. 
In birds with both sexes similar in appearance and with mutual court- 
ship, two salient facts are to be noted. In the first place, the birds are 
usually not protectively coloured, or at least not to such an extent as in 
most dimorphic species. Take for example the Herons and Egrets, the 
Swans, the Gulls, and many of the cliff-dwelling and other sea-birds 
(Guillemots, Razorbills, Petrels, &c.) . The Grebes and Divers are inter- 
mediate — the main coloration is protective, but the head, neck, and under- 
surface are conspicuous and used in courtship. In the second place, both 
sexes almost always share in nest-building, and often in incubation and 
the care of the young. So far as I know, there are no birds with mutual 
courtship in which the males do not share some of the duties of parent- 
hood, and their share is on the average certainly greater than is that of the 
males of birds with dimorphism in courtship. Also the nest-site is, usually 
at least, chosen by both birds jointly. 
It appears to be definitely the case that several tendencies are at work to 
produce the variation in modes of courtship that we see. (1) Where, as in 
the Warblers, &c., the whole life-cycle is based on the early staking-oiit 
of territory by the males, there must be a dimorphism of instinct in connection 
with this, and this primary dimorphism tends to be continued in other waj's, 
e. g. courtship. (2) Where protective coloration is of great importance to 
a species, it is imperative that one sex at least shall be thus protected ; this, 
sex will naturally, save in very exceptional cases, tend to be the female. If 
pi'otective coloration is desirable, it will almost always follow that general 
inconspicuousness will be desirable ; this will mean that any violent mani- 
festations of emotion, whether in the form of song or of disjjlay, will be 
undesirable. Hence there will be a tendency in such species to depress the 
level of sexual excitability in the females. This in its turn will, however, 
make it necessary for there to exist some mechanism for arousing their 
sexual emotion, and this will naturally be found in the courtship of the 
male. Thus in general the necessity for inconspicuousness in the female 
will have as natural reaction a tendency for courtship activities to be 
confined to the male, and to be well developed in him. Inconspicuous 
females tend to be associated with dimorphism of courtship, and often of 
colour. The tendencies mentioned in (1) and (2) often interact so that it is 
hard to distinguish which is the primary. Where polygamy prevails, and the 
males take no share in parental duties, this tendency will naturally produce 
its most marked results. (3) In some groups, such as for instance 
the Herons, not only is it necessary for both birds to share in feeding 
the young, but also to share the duties of incubation (since the feeding- 
LINN. JOXJRN.— ZOOLOGY, VOL. XXXV. 20 
