ting distribution of mountains and
plains, of rivers and seas, would show itself to be the product of a slow
process of natural change operating upon more and more widely different
antecedent conditions of the mineral framework of the earth; until, at
length, in place of that framework, he would behold only a vast nebulous
mass, representing the constituents of the sun and of the planetary
bodies. Preceding the forms of life which now exist, our observer would
see animals and plants, not identical with them, but like them; increasing
their difference with their antiquity and, at the same time, becoming
simpler and simpler; until, finally, the world of life would present
nothing but that undifferentiated protoplasmic matter which, so far as our
present knowledge goes, is the common foundation of all vital activity.

The hypothesis of evolution supposes that, in all this vast progression,
there would be no breach of continuity, no point at which we could say,
"This is a natural process," and, "This is not a natural process"; but
that the whole might be compared to that wonderful process of development
which may be seen going on every day under our eyes, in virtue of which
there arises, out of the semifluid, comparatively homogeneous substance
which we call an egg, the complicated organization of one of the higher
animals. That, in a few words, is what is meant by the hypothesis of
evolution.

I have already suggested that, in dealing with these three hypotheses, in
endeavoring to form a judgment as to which of them is the more worthy of
belief, or whether none is worthy of belief,--in which case our condition
of mind should be that suspension of judgment which is so difficult to all
but trained intellects,--we should be indifferent to all _a priori_
considerations. The question is a question of historical fact. The
universe has come into existence somehow or other, and the problem is,
whether it came into existence in one fashion, or whether it came into
existence in another; and, as an essential preliminary to further
discussion, permit me to say two or three words as to the nature and the
kinds of historical evidence.

The evidence as to the occurrence of any event in past time may be ranged
under two heads, which, for convenience' sake, I will speak of as
testimonial evidence and as circumstantial evidence. By testimonial
evidence I mean human testimony; and by circumstantial evidence I mean
evidence which is not hum