original slight resemblance, leading to occasional mistakes on the part
of birds, was increased generation after generation by the more frequent
escape of the most-like individuals, until the likeness became thus
great.

But now, recognizing in full this process brought into clear view by Mr.
Darwin, and traced out by him with so much care and skill, can we
conclude that, taken alone, it accounts for organic evolution? Has the
natural selection of favourable variations been the sole factor? On
critically examining the evidence, we shall find reason to think that it
by no means explains all that has to be explained. Omitting for the
present any consideration of a factor which may be distinguished as
primordial, it may be contended that the above-named factor alleged by
Dr. Erasmus Darwin and by Lamarck, must be recognized as a co-operator.
Utterly inadequate to explain the major part of the facts as is the
hypothesis of the inheritance of functionally-produced modifications,
yet there is a minor part of the facts, very extensive though less,
which must be ascribed to this cause.

* * * * *

When discussing the question more than twenty years ago (_Principles of
Biology_, Sec. 166), I instanced the decreased size of the jaws in the
civilized races of mankind, as a change not accounted for by the natural
selection of favourable variations; since no one of the decrements by
which, in thousands of years, this reduction has been effected, could
have given to an individual in which it occurred, such advantage as
would cause his survival, either through diminished cost of local
nutrition or diminished weight to be carried. I did not then exclude, as
I might have done, two other imaginable causes. It may be said that
there is some organic correlation between increased size of brain and
decreased size of jaw: Camper's doctrine of the facial angle being
referred to in proof. But this argument may be met by pointing to the
many examples of small-jawed people who are also small-brained, and by
citing not infrequent cases of individuals remarkable for their mental
powers, and at the same time distinguished by jaws not less than the
average but greater. Again, if sexual selection be named as a possible
cause, there is the reply that, even supposing such slight diminution of
jaw as took place in a single generation to have been an attraction, yet
the other incentives to choice on the part of men have be