this respect is to be attacked at all, it
can only be on grounds of _fact_--namely, by arguing that the cause does
not occur in nature, or that, if it does, its importance has been
exaggerated by the theory. Even, however, if the latter proposition
should ever be proved, we may now be virtually certain that the only
result would be the relegation of all the residual phenomena of
adaptation to other causes of the physical order--whether known or
unknown. Hence, as far as the matter of _principle_ is concerned, we may
definitely conclude that the great naturalistic movement of our century
has already brought all the phenomena of adaptation in organic nature
under precisely the same category of mechanical causation, as similar
movements in previous centuries have brought all the known phenomena of
inorganic nature: the only question that remains for solution is the
strictly _scientific_ question touching the particular causes of the
mechanical order which have been at work.

So much, then, for the phenomena of adaptation. Turning next to those of
beauty, we have already seen that the theory of sexual selection stands
to these in precisely the same relation as the theory of natural
selection does to those of adaptation. In other words, it supplies a
physical explanation of them; because, as far as our present purposes
are concerned, it may be taken for granted, or for the sake of argument,
that inasmuch as psychological elements enter into the question the
cerebral basis which they demand involves a physical side.

There is, moreover, this further point of resemblance between the two
theories: neither of them has any reference to inorganic nature.
Therefore, with the charm or the loveliness of landscapes, of earth and
sea and sky, of pebbles, crystals, and so forth, we have at present
nothing to do. How it is that so many inanimate objects are invested
with beauty--why it is that beauty attaches to architecture, music,
poetry, and many other things--these are questions which do not
specially concern the biologist. If they are ever to receive any
satisfactory explanation in terms of natural causation, this must be
furnished at the hands of the psychologist. It may be possible for him
to show, more satisfactorily than hitherto, that all beauty, whenever
and wherever it occurs, is literally "in the eyes of the beholder"; or
that objectively considered, there is no such thing as beauty. It may
be--and in my opinion it probably