rule amongst birds," remarks J.G. Millais (op.
cit., p. 46), "that they do not breed until both sexes have attained the
perfect adult plumage." Until that happens, it seems probable, the
conditions for sexual excitation are not fully established. We know
little, says Howard (_Zooelogist_, 1903, p. 407), of the age at which birds
begin to breed, but it is known that "there are yearly great numbers of
individuals who do not breed, and the evidence seems to show that such
individuals are immature."

[31] A. Marro, _La Puberte_, 1901, p. 464.

[32] Lloyd Morgan, _Animal Behavior_, 1900, pp. 264-5. It may be added
that, on the esthetic side, Hirn, in his study (_The Origins of Art_,
1900), reaches conclusions which likewise, in the main, concord with those
of Groos.

[33] It may be noted that the marriage ceremony itself is often of the
nature of a courtship, a symbolic courtship, embodying a method of
attaining tumescence. As Crawley, who has brought out this point, puts it,
"Marriage-rites of union are essentially identical with love charms," and
he refers in illustration to the custom of the Australian Arunta, among
whom the man or woman by making music on the bull-roarer compels a person
of the opposite sex to court him or her, the marriage being thus
completed. (E. Crawley, _The Mystic Rose_, p. 318.)

[34] The more carefully animals are observed, the more often this is found
to be the case, even with respect to species which possess no obvious and
elaborate process for obtaining tumescence. See, for instance, the
detailed and very instructive account--too long to quote here--given by E.
Selous of the preliminaries to intercourse practised by a pair of great
crested grebes, while nest-building. Intercourse only took place with much
difficulty, after many fruitless invitations, more usually given by the
female. ("Observational Diary of the Habits of the Great Crested Grebe,"
_Zoeologist_, September, 1901.) It is exactly the same with savages. The
observation of Foley (_Bulletin de la Societe d'Anthropologie de Paris_,
November 6, 1879) that in savages "sexual erethism is very difficult" is
of great significance and certainly in accordance with the facts. This
difficulty of erethism is the real cause of many savage practices which to
the civilized person often seem perverse; the women of the Caroline
Islands, for instance, as described by Finsch, require the tongue or even
the teeth to be applied to the clitoris, or a