The theory of evolution itself very often tends to become a metaphysical theory.  It does so when it holds the course of development which it traces to be either itself the ultimate reality or the most adequate appearance of that reality.  This theory is now commonly known by the name of Naturalism; according to it the facts dealt with by the natural sciences are the only reality which is knowable; man’s nature is part of these and has to be adapted to them, and there is nothing further with which it can be brought into relation.  This theory is not the same as the scientific theory of evolution, nor is it a necessary consequence of it; but in the minds of many the two go together.  The conclusion of the preceding argument—­that the ethical significance of evolution is not deep enough to give any answer to the fundamental question of morals—­is not a criticism of the theory of evolution so far as restricted to the domain of science, but it is a criticism of the Naturalism which professes to be a final philosophy.

III.

Ethics and idealism.

There has been no movement of metaphysical thought in our time which can be compared for its widespread influence or for its general acceptance with the theory of evolution in biological science.  Intimate as is its connexion with the progress of science, metaphysics does not keep step with it,—­any more than it simply marks time as the former advances.  It reflects the influence of each new generalisation of science; but if and so far as it reflects this influence only, it cannot be an adequate metaphysics.  Metaphysics must re-think each new fact brought to light, each new generalisation established by science.  It must think them in their relation to the whole, and attempt to understand them by setting them in their place in the complete system of knowledge and reality.  This complete system is indeed an ideal, never adequately comprehended by the human mind; but it is nevertheless the ideal which determines all efforts of constructive philosophy—­including those efforts which take the generalisation of some special science as their all-comprehending principle.  An attempt of this kind to make a philosophy out of a scientific generalisation has in our own time been the obvious result of the theory of evolution, and has given new vogue to the philosophical system called Naturalism.  That system draws its strength from the scientific doctrine of evolution; but as a philosophy it gives an extended application to the generalisation established by a group of sciences, and valid for the facts within their range.  It interprets the law of development which rules the sequences of nature as the highest attainable principle for explaining the system of things.  Some of the questions which it leaves unanswered, and some of the facts which it overlooks, have been pointed out in last lecture.  Of this theory perhaps enough has already been said.