And, in the third place, there are those who seek a solution which denies the truth of both the preceding, and which is metaphysically idealist or monist in character. To them, if the present writer understands their attitude, matter and spirit are different aspects of one reality. In the inorganic and non-living, phenomena appear which are generalized under the laws of physics and chemistry, but the phenomena of life fall into a different category which includes the conception of co-ordination or individuality, while a still higher category is required to include the phenomena of consciousness and mind. It is evident from this brief review that Biology in the period considered has passed through three main stages. The first of these was the acceptance of a new illuminating and unifying idea, which led to enthusiastic research in many directions for the purpose of proving and amplifying it. Very rapidly new facts, or new interpretations of facts already known, were shown to fall into line, and the evolution theory became converted from a hypothesis into something approaching a dogma. Not only the idea of organic evolution itself, but all the current beliefs about the method of evolution, and the larger speculations to which it gave rise, were widely regarded as almost indisputable, and where difficulties and inconsistencies appeared, these were supposed to be due solely to the insufficiency of our knowledge, which would soon be remedied. Then, however, as detailed knowledge increased, the voice of criticism and doubt was more frequently heard. The various branches of Biology began once more to overlap, and to join hands with chemistry and physics, and it became clear that the interpretation of life was very far from being a simple problem. And so, as with the Atomic Theory in chemistry, the present position is one of dissolution of the older ideas and of hesitation to express a fixed belief, for while Biology has a clearer vision of the problem before it than ever it had, its wider knowledge reveals the fact that the problem is far from being solved. Perhaps one of the chief results of the great increase of knowledge during the past sixty years has been to show us the immensity of the field still remaining to be explored. FOR REFERENCE Centenary volume on Darwin (Cambridge University Press). X ART A. CLUTTON-BROCK