, as in dogs, greater than those
on which distinctions of species have been founded. They can
shew, too, that the changes daily taking place in ourselves--the
facility that attends long practice, and the loss of aptitude
that begins when practice ceases,--the strengthening of the
passions habitually gratified, and the weakening of those
habitually curbed,--the development of every faculty, bodily,
moral, intellectual, according to the use made of it--are all
explicable on this principle. And thus they can shew that
throughout all organic nature there is at work a modifying
influence of the kind they assign as the cause of these specific
differences; an influence which, though slow in its action, does,
in time, if the circumstances demand it, produce marked
changes--an influence which, to all appearance, would produce in
the millions of years, and under the great varieties of condition
which geological records imply, any amount of change."

These and many other instances which might be brought together from
the published writings of the half-century before the publication of
the _Origin_, show conclusively that the idea of evolution was far
from new, and that all through the first part of this century
dissatisfaction with the doctrine of the fixity of species and of
their miraculous creation was growing. The great contribution of
Darwin was this: First, by his theory of natural selection, he brought
together the known facts of variation, of struggle for existence, and
of adaptation to varying conditions, in such a way that they provided
men with a rational and known cause, a cause the operation of which
could be seen, for the origin of species by means of preservation of
favoured races. Next, as to the origin of species, he brought together
not only proofs of the actual operation of natural selection, but a
body of evidence in favour of the fact of evolution that was, beyond
all comparison, more striking than had been adduced by any earlier
philosophical or biological writer. He convinced naturalists that
evolution was by far the most probable way in which the living world
had come to be what it is, and he made them turn to examination of the
animal and vegetable kingdoms with a lively hope that the past history
of the living world was not an insoluble problem. Darwin's doctrine
brought a new life into biological study, and the result of the
inco