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
kind observed.

Thus, to take another instance of apparent design from inorganic nature,
it has been argued that the constitution of the atmosphere is clearly
designed for the support of vegetable and animal life. But before this
conclusion can be established upon the facts, it must be shown that life
could exist under no other material conditions than those which are
furnished to it by the elementary constituents of the atmosphere. This,
however, it is clearly impossible to show. For anything that we can know
to the contrary, life may actually be existing upon some of the other
heavenly bodies under totally different conditions as to atmosphere; and
the fact that on this planet all life has come to be dependent upon the
gases which occur in our atmosphere, may be due simply to the fact that
it was only the forms of life which were able to adapt themselves
(through natural selection or other physical causes) to these particular
gases which could possibly be expected to occur--just as in matters of
still smaller detail, it was only those forms of life that were suited
to their several habitats in the marine