next to
notice that whichever of the two rival theories we choose to entertain,
we are not here concerned with any question touching the origin of life.
We are concerned only with the origin of particular forms of life--that
is to say, with the origin of species. The theory of descent starts from
life as a _datum_ already granted. How life itself came to be, the
theory of descent, as such, is not concerned to show. Therefore, in the
present discussion, I will take the existence of life as a fact which
does not fall within the range of our present discussion. No doubt the
question as to the origin of life is in itself a deeply interesting
question, and although in the opinion of most biologists it is a
question which we may well hope will some day fall within the range of
science to answer, at present, it must be confessed, science is not in a
position to furnish so much as any suggestion upon the subject; and
therefore our wisdom as men of science is frankly to acknowledge that
such is the case.

* * * * *

We are now in a position to observe that the theory of organic evolution
is strongly recommended to our acceptance on merely antecedent grounds,
by the fact that it is in full accordance with what is known as the
principle of continuity. By the principle of continuity is meant the
uniformity of nature, in virtue of which the many and varied processes
going on in nature are due to the same kind of method, i. e. the method
of natural causation. This conception of the uniformity of nature is
one that has only been arrived at step by step through a long and
arduous course of human experience in the explanation of natural
phenomena. The explanations of such phenomena which are first given are
always of the supernatural kind; it is not until investigation has
revealed the natural causes which are concerned that the hypotheses of
superstition give way to those of science. Thus it follows that the
hypotheses of superstition which are the latest in yielding to the
explanations of science, are those which refer to the more recondite
cases of natural causation; for here it is that methodical investigation
is longest in discovering the natural causes. Thus it is only by degrees
that fetishism is superseded by what now appears a common-sense
interpretation of physical phenomena; that exorcism gives place to
medicine; alchemy to chemistry; astrology to astronomy; and so forth.
Everywhere the mirac