ence of such a structure save for the
benefit of the species that presents it. Therefore, I say that this
immensely large and general fact speaks with literally immeasurable
force in favour of natural selection, as at all events one of the main
causes of organic evolution. For the fact is precisely what we should
expect if this theory is true, while upon no other theory can its
universality and invariability be rendered intelligible. On the
beneficent design theory, for instance, it is inexplicable that no
species should ever be found to present a structure or an instinct
having primary reference to the welfare of another species, when, _ex
hypothesi_, such an endless amount of thought has been displayed in the
creation of structures and instincts having primary reference to the
species which present them. For how magnificent a display of divine
beneficence would organic nature have afforded, if all--or even
some--species had been so inter-related as to have ministered to each
others wants. Organic species might then have been likened to a
countless multitude of voices, all singing in one great harmonious
psalm. But, as it is, we see absolutely no vestige of such
co-ordination: every species is for itself, and for itself alone--an
outcome of the always and everywhere fiercely raging struggle for life.

In order that the force of this argument may not be misapprehended, it
is necessary to bear in mind that it is in no way affected by cases
where a structure or an instinct is of primary benefit to its possessor,
and then becomes of secondary benefit to some other species on account
of the latter being able in some way or another to utilise its action.
Of course organic nature is full of cases of this kind; but they only go
to show the readiness which all species display to utilise for
themselves everything that can be turned to good account in their own
environments, and so, among other things, the structures and instincts
of other animals. For instance, it would be no answer to Darwin's
challenge if any one were to point to a hermit-crab inhabiting the
cast-off shell of a mollusk; because the shell was primarily of use to
the mollusk itself, and, so far as the mollusk is concerned, the fact of
its shell being afterwards of a secondary use to the crab is quite
immaterial. What Darwin's challenge requires is, that some structure or
instinct should be shown which is not merely of such secondary or
accidental benefit to anot