COURTSHIP ACTIVITIES IN THE RED-THROATED DIVER. 287 
characters arc special structures only fully displayed during courtship (e. g. 
scapulars of Egrets, tail-coverts of Peacock, rufF of Crested Grebe) ; others 
are special patches of colour only made prominent by special display actions 
[e.g., white on wings of Blackcock and of Crested Grrebe, red of legs of Red- 
shank, rich purplish-brown of neck of Louisiana Heron) ; while still others, 
although always present, are probably not prominent unless the two birds are 
close to each other, and in certain relative positions characteristic of courtship. 
This is probably the case with many of the curious patterns characterizing the 
front vieio of birds at short range [e. g. Blue Tit, Turnstone, Redstart, Ringed 
Plover) ; that this is the probable explanation is shown b}' the fact that when 
special structures with undoubted epigamic function are developed on the 
head, these are often displayed so as to appear most striking (or only so) 
when seen in this way, close up and from the front {e. g. the ruff and ear- 
tufts of the Crested Grebe, the crest of Louisiana Heron and Little Egret). 
The existence of true sexual selection as found in polygamous species thus 
encourages the same tendencies in epigamic characters as does the selection 
of those with a purely "accessory" function ; but, owing to the fact that in 
polygamous species the males take no share in incubation or the care of the 
young, dimorphism can proceed to its limit, and owing to the fact that there 
is a real selection as between different males, and so greater competition in 
regard to secondary sexual characters, and that the successful male transmits 
his qualities to a greater number of offspring, the process of evolution of 
epigamic characters is not only more rapid, but also is generally carried to a 
higher pitch than in monogamous species. In other words, polygamy and its 
attendant true sexual selection simply accentuate the same pi-ocesses that ai-e 
operative whenever epigamic characters are being evolved, even in the 
absence of sexual selection proper. 
This development of epigamic characters in relation to the mind of the 
opposite sex is a point of very general biological interest, since it is the only 
example, in organisms below man, of a secondary effect of the mind of a 
species upon the evolution of that species. Mental qualities of course 
normally have survival value, but this is quite a different matter ; they thus 
determine the survival or extinction of the species, not its modification in new 
ways. The mind of one species may play a part in moulding the evolutionary 
development of other species, as when acute vision on the part of predaceous 
animals renders concealing coloration advantageous, or the visual and olfactory 
preferences of flower- visiting insects are reflected in the development of colour 
and scent in the flowers visited. Lloyd Morgan ('21) has recently emphasized 
the influence of mind upon evolution by introducing the useful term " Psychical 
Selection." As indicated above, however, further terms are needed to 
distinguish between psychical selection acting upon the evolution of other 
species, as in the case of the mental qualities of bees influencing the evolution 
of flowers, or upon the evolution of the same species, as in the development 
