as it is
obviously subject to variation; just as it must be enhanced under
natural selection, whether individually acquired increments are
inherited or not; and just as its value lies not only in this or that
special perceptive act but in its importance for life as a whole; so
the vigourous effectiveness of activity has survival-value; it is
subject to variation; it must be enhanced under natural selection; and
its importance lies not only in particular modes of behaviour but in
its value for life as a whole. If emotion and its expression as a
congenital endowment are but different aspects of the same biological
occurrence; and if this is a powerful supplement to vigour
effectiveness and persistency of behaviour, it must on Darwin's
principles be subject to natural selection.

If we include under the expression of the emotions not only the
premonitory symptoms of the initial phases of the organic and mental
state, not only the signs or conditions of half-tide emotion, but the
full-tide manifestation of an emotion which dominates the situation,
we are naturally led on to the consideration of many of the phenomena
which are discussed under the head of sexual selection. The subject is
difficult and complex, and it was treated by Darwin with all the
strength he could summon to the task. It can only be dealt with here
from a special point of view--that which may serve to illustrate the
influence of certain mental factors on the course of evolution. From
this point of view too much stress can scarcely be laid on the
dominance of emotion during the period of courtship and pairing in the
more highly organised animals. It is a period of maximum vigour,
maximum activity, and, correlated with special modes of behaviour and
special organic and visceral accompaniments, a period also of maximum
emotional excitement. The combats of males, their dances and aerial
evolutions, their elaborate behaviour and display, or the flood of
song in birds, are emotional expressions which are at any rate
coincident in time with sexual periodicity. From the combat of the
males there follows on Darwin's principles the elimination of those
which are deficient in bodily vigour, deficient in special structures,
offensive or protective, which contribute to success, deficient in the
emotional supplement of which persistent and whole-hearted fighting is
the expression, and deficient in alertness and skill which are the
outcome of the psychological develop