on, into the
presence of prey of greater speed than before; then, while all its
members will have their limbs strengthened by extra action, those in
whom this muscular adaptation is greatest will have their multiplication
furthered; and inheritance of the functionally-increased structures will
be aided, in successive generations, by survival of the fittest. But it
cannot be so with the multitudinous minor changes entailed by the
modified life. The majority of these must be of such relative
unimportance that one of them cannot give to the individual in which it
becomes most marked, advantages which predominate over kindred
advantages gained by other individuals from other changes more
favourably wrought in them. In respect to these, the inherited effects
of use and disuse must accumulate independently of natural selection.

To make clear the relations of these two factors to one another and to
heredity, let us take a case in which the operations of all three may be
severally identified and distinguished.

Here is one of those persons, occasionally met with, who has an
additional finger on each hand, and who, we will suppose, is a
blacksmith. He is neither aided nor much hindered by these additional
fingers; but, by constant use, he has greatly developed the muscles of
his right arm. To avoid a perturbing factor, we will assume that his
wife, too, exercises her arms in an unusual degree: keeps a mangle, and
has all the custom of the neighbourhood. Such being the circumstances,
let us ask what are the established facts, and what are the beliefs and
disbeliefs of biologists.

The first fact is that this six-fingered blacksmith will be likely to
transmit his peculiarity to some of his children; and some of these,
again, to theirs. It is proved that, even in the absence of a like
peculiarity in the other parent, this strange variation of structure
(which we must ascribe to some fortuitous combination of causes) is
often inherited for more than one generation. Now the causes which
produce this persistent six-fingeredness are unquestionably causes which
"operate through inheritance." The Duke of Argyll says that "natural
selection includes and covers all the causes which can possibly operate
through inheritance." How does it cover the causes which operate here?
Natural selection never comes into play at all. There is no fostering of
this peculiarity, since it does not help in the struggle for existence;
and there is no reaso