hesis of descent.

Thus it is clear that the _a priori_ standing of the rival hypotheses of
naturalism and supernaturalism in the case of all these pieces of
organic machinery, is profoundly affected by the question whether they
came into existence suddenly, or whether they did so gradually. For, if
they all came into existence suddenly, the fact would constitute
well-nigh positive proof in favour of supernaturalism, or creation by
design; whereas, if they all came into existence gradually, this fact
would in itself constitute presumptive evidence in favour of naturalism,
or of development by natural causes. And, as shown in the previous
chapters, the proof that all species of plants and animals came into
existence gradually--or the proof of evolution as a fact--is simply
overwhelming.

From a still more general point of view I may state the case in another
way, by borrowing and somewhat expanding an illustration which, I
believe, was first used by Professor Huxley. If, when the tide is out,
we see lying upon the shore a long line of detached sea-weed, marking
the level which is reached by full tide, we should be free to conclude
that the separation of the sea-weed from the sand and the stones was due
to the intelligent work of some one who intended to collect the sea-weed
for manure, or for any other purpose. But, on the other hand, we might
explain the fact by a purely physical cause--namely, the separation by
the sea-waves of the sea-weed from the sand and stones, in virtue of its
lower specific gravity. Now, thus far the fact would be explained
equally well by either hypothesis; and this fact would be the fact of
_selection_. But whether we yielded our assent to the one explanation or
to the other would depend upon a due consideration of all collateral
circumstances. The sea-weed might not be of a kind that is of any use to
man; there might be too great a quantity of it to admit of our supposing
that it had been collected by man; the fact that it was all deposited on
the high-water-mark would in itself be highly suggestive of the agency
of the sea; and so forth. Thus, in such a case any reasonable observer
would decide in favour of the physical explanation, or against the
teleological one.

Now the question whether organic evolution has been caused by physical
agencies or by intelligent design is in precisely the same predicament.
There can be no logical doubt that, theoretically at all events, the
physical agenc