I trust that I have now made it sufficiently clear why I hold that if we
believe the reign of natural law, or the operation of physical causes,
to extend throughout organic nature in the same universal manner as we
believe this in the case of inorganic nature, then we can find no better
evidence of design in the one province than in the other. The mere fact
that we meet with more numerous and apparently more complete instances
of design in the one province than in the other is, _ex hypothesi_,
merely due to our ignorance of the natural causation in the more
intricate province. In studying biological phenomena we are all at
present in the intellectual position of our imaginary teleologist when
studying the marine bay: we do not know the natural causes which have
produced the observed results. But if, after having obtained a partial
key in the theory of natural selection, we trust to the large analogy
which is afforded by the simpler provinces of Nature, and conclude that
physical causes are everywhere concerned in the production of organic
structures, then we have concluded that any evidence of design which
these structures present is of just the same logical value as that which
we may attach to the evidence of design in inorganic nature. If it
should still be urged that the adaptations met with in organic nature
are from their number and unity much more suggestive of design than
anything met with in inorganic nature, I must protest that this is to
change the ground of argument and to evade the only point in dispute.
No one denies the obvious fact stated: the only question is whether any
number and any quantity of adaptations in any one department of nature
afford other or better evidence of design than is afforded by
adaptations in other departments, when all departments alike are
supposed to be equally the outcome of physical causation. And this
question I answer in the negative, because we have no means of
ascertaining the extent to which the process of natural selection, or
any other physical cause, is competent to produce adaptations of the