insight of others had passed unperceived. Since Darwin wrote the _Descent
of Man_ the chief step in the development of the theory of sexual
selection has been taken by Groos, who has at the same time made it clear
that sexual selection is largely a special case of natural selection.[29]
The conjunction of the sexes is seen to be an end only to be obtained with
much struggle; the difficulty of achieving sexual erethism in both sexes,
the difficulty of so stimulating such erethism in the female that her
instinctive coyness is overcome, these difficulties the best and most
vigorous males,[30] those most adapted in other respects to carry on the
race, may most easily overcome. In this connection we may note what Marro
has said in another connection, when attempting to answer the question why
it is that among savages courtship becomes so often a matter in which
persuasion takes the form of force. The explanation, he remarks, is yet
very simple. Force is the foundation of virility, and its psychic
manifestation is courage. In the struggle for life violence is the first
virtue. The modesty of women--in its primordial form consisting in
physical resistance, active or passive, to the assaults of the male--aided
selection by putting to the test man's most important quality, force. Thus
it is that when choosing among rivals for her favors a woman attributes
value to violence.[31] Marro thus independently confirms the result
reached by Groos.

The debate which has for so many years been proceeding concerning the
validity of the theory of sexual selection may now be said to be brought
to an end. Those who supported Darwin and those who opposed him were, both
alike, in part right and in part wrong, and it is now possible to combine
the elements of truth on either side into a coherent whole. This is now
beginning to be widely recognized; Lloyd Morgan,[32] for instance, has
readjusted his position as regards the "pairing instinct" in the light of
Groos's contribution to the subject. "The hypothesis of sexual selection,"