I have now recapitulated the facts and considerations which
have thoroughly convinced me that species have been modified during a
long course of descent. This has been effected chiefly through the
natural selection of numerous successive, slight, favourable variations;
aided in an important manner by the inherited effects of the use and
disuse of parts; and in an unimportant manner--that is, in relation to
adaptive structures whether past or present, by the direct action of
external conditions, and by variations which seem to us, in our
ignorance, to arise spontaneously." This passage, summarising Darwin's
whole inquiry, and explaining his final point of view, shows how very
inaccurate may be the popular notion, as expressed by the Duke of
Argyll, of any supposed additions to the causes of change of species as
recognised by Darwin.

But, as we shall see presently, there is now much reason to believe
that the supposed inheritance of acquired modifications--that is, of the
effects of use and disuse, or of the direct influence of the
environment--is not a fact; and if so, the very foundation is taken away
from the whole class of objections on which so much stress is now laid.
It therefore becomes important to inquire whether the facts adduced by
Darwin, Spencer, and others, do really necessitate such inheritance, or
whether any other interpretation of them is possible. I believe there is
such an interpretation; and we will first consider the cases of disuse
on which Mr. Spencer lays most stress.

The cases Mr. Spencer adduces as demonstrating the effects of disuse in
diminishing the size and strength of organs are, the diminished size of
the jaws in the races of civilised men, and the diminution of the
muscles used in closing the jaws in the case of pet-dogs fed for
generations on soft food. He argues that the minute reduction in any one
generation could not possibly have been useful, and, therefore, not the
subject of natural selection; and against the theory of correlation of
the diminished jaw with increased brain in man, he urges that there are
cases of large brain development, accompanied by jaws above the average
size. Against the theory of economy of nutrition in the case of the
pet-dogs, he places the abundant food of these animals which would
render such economy needless.

But neither he nor Mr. Darwin has considered the effects of the
withdrawal of the action of natural selection in keeping up the parts in
que