From what has been said it will be evident that, fundamentally, all biological research, whether its authors are conscious of it or not, is directed towards the solution of one central problem--the problem of the real and ultimate nature of life. And the main outcome of the work of sixty years has been that this problem has begun clearly to emerge as the central aim of the science. The theory of evolution made the problem a reality, for without evolution the mystery of life must for ever be insoluble, but whatever direction biological investigation has taken, it has led, often by devious paths, to the borderland between the living and the inorganic, and in that borderland the central problem inevitably faces us. Many suggestions for its solution have been made. On the one hand there is still, as there always has been, a considerable body of opinion that the solution will be a mechanical one--using the word mechanical in the widest sense--and that the living differs from the non-living not in kind, but only in degree of complexity. The upholders of the mechanistic or materialist theory, however, are perhaps less confident than their predecessors of the last century, for the solution in this direction has to face not only the problem of organic co-ordination already referred to, but also that of consciousness and mind. For although the study of psychology on physiological lines has made similar progress to that of other branches of physiology, it seems to approach little nearer to a discovery of the nature of the relation between consciousness in its various aspects and the material body with which it is associated. So long as this gulf remains unbridged, the possibility of a satisfactory mechanistic explanation of life seems far away. On the other hand, there has been a revival of the ancient tendency towards what is called a vitalistic solution. A certain number of biologists, impressed by the apparent similarity between the control and co-ordination exercised by the organism over its functions and the conscious control of voluntary activity with which we are familiar in ourselves, have supposed that these things are not merely superficially similar but have a real and fundamental affinity. This does not mean that organic control is always conscious, but that there is a controlling entity, non-material in nature, which is similar in kind to the 'ego' of a self-conscious human being. They suppose that the organism is not simply material, but is a material mechanism controlled by a non-material entity the nature of which is more akin to what we mean by the word spirit than anything else of which we are accustomed to think. They are in fact dualists, and divide reality into the material and spatial on the one hand, and non-material principle or entity which may fairly be called spiritual on the other.