that I have made the evidences of evolution as a fact to
stand independently on their own feet--feet which in my opinion are
amply strong enough to bear any weight of adverse criticism that can be
placed upon them.

Our position, then, is this. On the foundation of the previous chapters,
I will henceforth assume that we all accept organic evolution as a fact,
without requiring any of the accessory evidence which is gained by
independent proof of natural selection as a method. But in making this
assumption--namely, that we are all now firmly persuaded of the fact of
evolution--I do not imagine that such is really the case. I make the
assumption for the purposes of systematic exposition, and in order that
different parts of the subject may be kept distinct. I confess it does
appear to me remarkable that there should still be a doubt in any
educated mind touching the general fact of evolution; while it becomes
to me unaccountable that such should be the case with a few still living
men of science, who cannot be accused of being ignorant of the evidences
which have now been accumulated. But in whatever measure we may
severally have been convinced--or remained unconvinced--on this matter,
for the purposes of exposition I must hereafter assume that we are all
agreed to the extent of regarding the process of evolution as, at least,
sufficiently probable to justify enquiry touching its causes on
supposition of its truth.

Now, the causes of evolution have been set forth in a variety of
different hypotheses, only the chief of which need be mentioned here.
Historically speaking the first of these was that which was put forward
by Erasmus Darwin, Lamarck, and Herbert Spencer. It consists in putting
together the following facts and inferences.

We know that, in the lifetime of the individual, increased use of
structures leads to an increase of their functional efficiency; while,
on the other hand, disuse leads to atrophy. The arms of a blacksmith,
and the legs of a mountaineer, are familiar illustrations of the first
principle: our hospital wards are full of illustrations of the second.
Again, we know that the characters of parents are transmitted to their
progeny by means of heredity. Now the hypothesis in question consists
in supposing that if any particular organs in a species are habitually
used for performing any particular action, they must undergo a
structural improvement which would more and more adapt them to the
perfo