For, in truth, it is Biology above all other sciences which has profited by the doctrine of evolution. In evolution, - in the doctrine that the whole cosmical order is the outcome of a gradual development, - mankind have gained for the first time a working hypothesis which covers enough of the known facts of the universe to make its possible extension to all facts a matter of hopeful interest. And Biology, which even at the date of Whewell's book could barely make good its claim to be regarded as a coherent science at all, has now acquired a co-ordinating and continuous principle of unity which renders it in some respects the best type of a true science which we possess. It traces life from the protozoon to the animal, from the brute to the man; it offers to explain the complex fabric of human thought and emotion, viewed from the physical side, as the development of the molecular movements of scarcely-differentiated fragments of protoplasm.

And along with this increased knowledge of the processes by which man has been upbuilt has come also an increased knowledge of the processes which are now going on within him. The same inquiries which have brought our organic life into intelligible relation with the whole range of animal and vegetable existence have enabled us also to conceive more definitely the neural side of our mental processes, and the relation of cerebral phenomena to their accompanying emotion or thought. And hence, in the view of some ardent physiologists, it is becoming more and more probable that we are in fact physiological automata; that our consciousness is a mere superadded phenomenon - a mere concomitant of some special intensity of cerebral action, with no basis beyond or apart from the molecular commotion of the brain.

But this view, as it would seem, depends in a great part upon something which corresponds in the mental field to a familiar optical illusion. When we see half of some body strongly illuminated, and half of it feebly illuminated, it is hard to believe that the brilliant moiety is not the larger of the two. And, similarly, it is the increased definiteness of our conception of the physical side of our mental operations which seems to increase its relative importance, - to give it a kind of priority over the psychical aspect of the same processes. Yet, of course, to the philosophic eye the central problem of the relation of the objective and subjective sides of these psycho-neural phenomena can be in no way altered by any increase of definiteness in our knowledge of the objective processes which correspond to the subjective states.

And, on the other hand, there is one singular logical corollary which seems thus far to have escaped the notice of physiologist and psychologist alike. It is this: that our increased vividness of conception of the physical side of mental life, while it cannot possibly disprove the independence of the psychical side, may quite conceivably prove it. I will again resort to the (very imperfect) analogy of a partially illuminated body. Suppose that one hemisphere of a globe is strongly lit up, and that the other is lit up by faint and scattered rays.1 I am trying to discern whether the two hemispheres are symmetrically marked throughout. Now no clearness of marks on the bright hemisphere can disprove the existence of corresponding marks on the dim one. But, on the other hand, it is conceivable that one of the few rays which fall on the dim hemisphere may reveal some singular mark which I can see that the bright hemisphere does not possess. And the brighter the bright hemisphere is made, the more certain do I become that this particular mark is not to be found on it.

§ 5. I will give two concrete examples of what I mean - one of them drawn from the conclusions of a great physiologist, the other from the obvious condition of a new branch of experimental inquiry. I shall not discuss either instance in detail, since I am here only endeavouring to show that with increased precision in psycho-physical researches the old problems of free-will, soul and body, etc., are presenting more definite issues, and offering a far more hopeful field to the exact philosopher than their former vagueness allowed.

1 The analogy will bo closer if we suppose that the second half is lit, not dimly but from within, - since in one sense consciousness gives us more information as to the psychical than as to the physical side of life, though it is information of a different.

My first illustration, then, is from the form which the old free-will controversy has assumed in the hands of Wundt. Wundt stands, of course, among the foremost of those who have treated human thought and sensation as definite and measurable things, who have computed their rate of transit, and analysed their elements, and enounced the laws of their association. It is not from him that we need look for any lofty metaphysical view as to the infinite resources of spiritual power, - the transcendental character of psychical phenomena. But, nevertheless, Wundt believes himself able to assert that there is within us a residue - an all-important residue - of psychical action which is incommensurable with physiological law. So far, he holds, is the principle of conservation of energy from covering the psychical realm, that the facts of mental evolution proclaim that the very contrary is the case; - and that what really obtains is rather " an unlimited new creation of psychical energy."1 Nay, so convinced is he of the inadequacy of any system of physiological determinism to explain psychical facts, that he holds that we must directly reverse the materialistic view of the relation of the corporeal to the psychical life. "It is not the psychical life," he says, "which is a product of the physical organisation; rather it is the physical organism which, in all those purposive adjustments which distinguish it from inorganic compounds, is itself a psychical creation."2