; so far and no farther. For Darwin, says Mr Wallace
(Ibid. page 283.), "has extended the principle into a totally different
field of action, which has none of that character of constancy and of
inevitable result that attaches to natural selection, including male
rivalry; for by far the larger portion of the phenomena, which he
endeavours to explain by the direct action of sexual selection, can only
be so explained on the hypothesis that the immediate agency is female
choice or preference. It is to this that he imputes the origin of
all secondary sexual characters other than weapons of offence and
defence... In this extension of sexual selection to include the action of
female choice or preference, and in the attempt to give to that choice
such wide-reaching effects, I am unable to follow him more than a very
little way."

Into the details of Mr Wallace's criticisms it is impossible to enter
here. We cannot discuss either the mode of origin of the variations in
structure which have rendered secondary sexual characters possible
or the modes of selection other than sexual which have rendered them,
within narrow limits, specifically constant. Mendelism and mutation
theories may have something to say on the subject when these theories
have been more fully correlated with the basal principles of selection.
It is noteworthy that Mr Wallace says ("Darwinism", pages 283, 284.):
"Besides the acquisition of weapons by the male for the purpose of
fighting with other males, there are some other sexual characters which
may have been produced by natural selection. Such are the various sounds
and odours which are peculiar to the male, and which serve as a call to
the female or as an indication of his presence. These are evidently a
valuable addition to the means of recognition of the two sexes, and
are a further indication that the pairing season has arrived; and the
production, intensification, and differentiation of these sounds and
odours are clearly within the power of natural selection. The same
remark will apply to the peculiar calls of birds, and even to the
singing of the males." Why the same remark should not apply to their
colours and adornments is not obvious. What is obvious is that "means
of recognition" and "indication that the pairing season has arrived" are
dependent on the perceptive powers of the female who recognises and for
whom the indication has meaning. The hypothesis of female preference,
stripped of the aesthe