ed as
a matter of fact; the selective influence of conditions, which no one
could deny to be a matter of fact, when his attention was drawn to the
evidence; and the occurrence of great geological changes which also
was matter of fact; could be used as the only necessary postulates of
a theory of the evolution of plants and animals which, even if not at
once, competent to explain all the known facts of biological science,
could not be shown to be inconsistent with any. So far as biology is
concerned, the publication of the 'Origin of Species,' for the first
time, put the doctrine of evolution, in its application to living
things, upon a sound scientific foundation. It became an instrument of
investigation, and in no hands did it prove more brilliantly
profitable than in those of Darwin himself. His publications on the
effects of domestication in plants and animals, on the influence of
cross-fertilisation, on flowers as organs for effecting such
fertilisation, on insectivorous plants, on the motions of plants,
pointed out the routes of exploration which have since been followed
by hosts of inquirers, to the great profit of science.

Darwin found the biological world a more than sufficient field for
even his great powers, and left the cosmical part of the doctrine to
others. Not much has been added to the nebular hypothesis, since the
time of Laplace, except that the attempt to show (against that
hypothesis) that all nebulae are star clusters, has been met by the
spectroscopic proof of the gaseous condition of some of them.
Moreover, physicists of the present generation appear now to accept
the secular cooling of the earth, which is one of the corollaries of
that hypothesis. In fact, attempts have been made, by the help of
deductions from the data of physics, to lay down an approximate limit
to the number of millions of years which have elapsed since the earth
was habitable by living beings. If the conclusions thus reached should
stand the test of further investigation, they will undoubtedly be very
valuable. But, whether true or false, they can have no influence upon
the doctrine of evolution in its application to living organisms. The
occurrence of successive forms of life upon our globe is an historical
fact, which cannot be disputed; and the relation of these successive
forms, as stages of evolution of the same type, is established in
various cases. The biologist has no means of determining the time over
which the pro