by natural selection has reference
to the degree in which these observable facts, when thus brought
together, are adequate to account for the process of evolution.

* * * * *

So much, then, as a statement of the theory of natural selection. But
from this statement--i. e. from the theory of natural selection
itself--there follow certain matters of general principle which it is
important to bear in mind. These, therefore, I shall here proceed to
mention.

First of all, it is evident that the theory is applicable as an
explanation of organic changes in specific types only in so far as these
changes are of _use_, or so far as such changes endow the species with
better chances of success in the general struggle for existence. This is
the only sense in which I shall always employ the terms use, utility,
service, benefit, and so forth--that is to say, in the sense of
life-preserving.

* * * * *

Next, it must be clearly understood that the life which it is the
object, so to speak, of natural selection to preserve, is primarily the
life of the _species_; not that of the _individual_. Natural selection
preserves the life of the individual only in so far as this is conducive
to that of the species. Wherever the life-interests of the individual
clash with those of the species, that individual is sacrificed in favour
of others who happen better to subserve the interests of the species.
For example, in all organisms a greater or less amount of vigour is
wasted, so far as individual interests are concerned, in the formation
and the nourishment of progeny. In the great majority of plants and
animals an enormous amount of physiological energy is thus expended.
Look at the roe or the milt of a herring, for instance, and see what a
huge drain has been made upon the individual for the sake of its
species. Again, all unselfish instincts have been developed for the sake
of the species, and usually against the interests of the individual. An
ant which will allow her head to be slowly drawn from her body rather
than relinquish her hold upon a pupa, is clearly acting in response to
an instinct which has been developed for the benefit of the hive, though
fatal to the individual. And, in a lesser degree, the parental
instincts, wherever they occur, are more or less detrimental to the
interests of the individual, though correspondingly essential to those
of the race.

These i