COURTSHIP ACTIVITIES IN THE KED-THROATED DIVER. 269 
ol nest-material in the first instance, might have been developed in 
association with the pairing-platform to a much greater pitch than they 
had ever been formerly. If this is so, and oilhand I am inclined to 
accept it as the more probable, there has been an evolution of the pairing- 
platform and the ceremonies connected with it, which is in many ways 
similar to that of the Bower-birds' bower. 
Personally I have not seen nearly so many post-mating ceremonies in 
this species as are to be observed in the Crested Grebe. This is undoubtedly 
due in part to the paucity of my obsei:vations. The work of Selous shows 
that " Penguin " ceremonies do occur in this period, and, it is interesting 
to note, continue, though rarely, after the young are hatched. Thus the 
physiological state (presumably dependent on gonadial secretion) during 
which epigamic ceremonies can be performed extends, appiarently in all 
birds, up to the very end of the breeding-season ; it does not then issue so 
frequently in " courtship," because incubation and the care of the youig 
afford other outlets for ph) sical energy and emotional tension. 
On the other hand, only three, or possibly four types of epigamic ceremony, 
whether pre- or post-mating, have been observed at all in this species, 
whereas in the Grebe six have been seen ; and the greater abundance in 
the latter is probably not only apparent but real, and associated wilh the 
greater development of epigamic ornament in the Grebe. 
Incubation appears to be undertaken mainly by one bird, the smalU'rof the 
pair ; but there are numerous records of males also incubating, and it is 
probable that in the Divers, as in other forms with mutual courtship, there is 
a sharing of the duties of incubation as well as those of caring for the young. 
It is noteworthy that no ceremonies associated with nest-relief have been- 
observed in the Divers or Grebes, whereas they are common in Herons (my 
own observations), Pelicans (Chapman, '08), &c. It would be important to 
find out the range of size in male and female Divers. I am inclined to 
believe that a single bimodal curve would be obtained. For a discussion of 
sex-difference in birds with close similarity of male and female, see Huxley, 
'22 b. 
General Discussion. 
It remains to discuss some of the general significance of such forms of 
mutual courtship as are seen in the Divers, Grebes, Herons, and Egrets — to 
mention only birds whose sexual habits I have mj^self investigated. 
As I have recently pointed out (Huxley, '21), it is impossible in the 
present state of our knowledge to maintain that Darwin's original theory of 
sexual selection is adequate to explain the origin of most of the sexual 
ceremonies and adornments to be found in monogamous birds. These 
adornments are chiefly used in ceremonies which take place after mating-up 
has taken place for the season. There cannot therefore be a direct selection 
as between one male and another in respect to them. It may be mentioned 
