If from the consideration of the isolated spinal animal we turn for a moment to that of the isolated cerebral animal, we find it in a very different position. It is possessed of the warning senses--those which from several points of view are the leading senses--but they are leaders without a following; they have only a very limited company to conduct into action. The company is there, on the further side of the severance, but they have lost touch with it. They know not what it is doing, and have therefore neither the data nor the executive power to guide its manœuvres in the field of behaviour. They can form maimed coalescent situations, but they are as impotent as a mere theorist devoid of all power of practical application. We need not, however, follow the theme further. We need only add that, could we isolate tracts of nervous tissue in the lower brain-centres of such a cerebral animal, we should find that subsidiary co-ordinations would belong, as a physiological heritage, to these isolated fragments. The conclusions we may draw, then, with regard to the evolution of behaviour, as viewed in its physiological aspect, are that it is, in its simplest expression and in its most complex, conditioned by sufficient unity of purpose to meet the biological end of survival; that the complex unity of purpose may be analyzed into a multiplicity of subsidiary processes each with its subsidiary unity of purpose; and that the psychological coalescence which gives unity to experience under the guidance of the leading senses, is paralleled in a physiological coalescence within the nexus of the nervous system. II.--THE BIOLOGICAL ASPECT The biological aspect of behaviour--its relation to biological ends--has so often come under our consideration in the foregoing chapters that little need be added in this section: and that little may be most appropriately devoted, first to the question whether consciousness does influence behaviour; and secondly, this being accepted, to the importance of the _rôle_ that is played by the development of conscious situations in securing, in the higher animals, the biological end of racial preservation.