COURTSHIP OF 'THE GREAT CRESPED GREBE, 527 
this term always implies a difference between the two sexes, and 
yet the crest of the Grebe has a sort of secondary sexual look 
about it—unreflectingly, one would at once write it down as 
such. This is due to our incomplete classification. We begin 
by separating out sexual characters from all others—these being 
characters that are different in the two sexes. We divide them 
into primary, accessory, and secondary. The mammee of mammals 
(with the exception of man) have nothing to do with courtship 
or mating, yet they are usually included under the same heading 
as the tail of the Peacock, while the Grebes’ courtship-structures 
would be left out in the cold. 
Besides the mere criterion of difference in the two sexes, we 
must have some other criterion—a criterion of use. 
It is naturally impossible to draw up any completed classifi- 
cation that will satisfy every case. To do so would be beyond 
the powers even of a Herbert Spencer—and not of much use when 
done. 
It is enough to point out, first, that our group of Secondary 
Sexual Characters is a bit of an omniwm gatherum. Some of 
them, as the mimicry of the female Papilio, or the brown colour 
of the female Pheasant, are protective, of use to the individual 
and to her offspring. Others, such as the mammee of mammals, 
are of use only to the offspring; others, like the sexual differ- 
ences in the beak of the Huia, where male and female hunt in 
couples, one splitting open the wood, the other picking out the 
hidden grubs, have arisen by a division of labour, and are of use 
to the couple as a couple. One might go on, but it would be 
unprofitable. 
In the second place, we must recognize as a fact that the 
existence of individuals of separate sexes with wills of their own 
has led to the development of what we call courtship—simply a 
process in which a series of actions is carried out as the outcome 
of an emotional state based on sexual excitement. All courtship 
is based on sexual excitement, and characters connected with 
courtship merit a separate name of their own. This name lies 
ready to hand in Poulton’s term epigamic; we must, however, 
remember that the literal meaning of the term must not be 
pressed, for in many cases the courtship ceremonies do not lead, 
directly or indirectly, to the act of pairing. Let us rather turn 
it the other way about, and, defining an epigamic character as one 
that is used in courtship, go on to define courtship as a series of 
actions based immediately or remotely upon sexual excitement, 
and, to make ourselves clear, we must add that sexual excitement 
is not merely sexual desire, but that whole emotional state into 
which a member of one sex may be thrown by a member of 
the other. The necessity for the distinction is obvious, if 
we think of the conditions in Man. Sexual excitement, of 
course, includes mere sexual desire, and also includes the fighting 
of males among each other as a result of sexual desire. 
