Whatever may be one’s view as to the larger pretensions that are put forward as to the significance and destiny of man, there can be no doubt {243} that it is indispensable to recognize the full implications of his status as an animal completely indigenous in the zoological series. The whole of his physical and mental structure is congruous with that of other living beings, and is constantly giving evidence of the complicated network of relationships by which he is bound to them. The accumulation of knowledge is steadily amplifying the range over which this congruity with the natural order can be demonstrated, and is showing more and more fully that practical understanding and foresight of man’s behaviour are attained in proportion as this hypothesis of the complete “naturalness” of man is adhered to. The endowment of instinct that man possesses is in every detail cognate with that of other animals, provides no element that is not fully represented elsewhere, and above all—however little the individual man may be inclined to admit it—is in no degree less vigorous and intense or less important in relation to feeling and activity than it is in related animals. This supremely important side of mental life, then, will be capable of continuous illustration and illumination by biological methods. It is on the intellectual side of mental life that man’s congruity with other animals is least obvious at first sight. The departure from type, however, is probably a matter of degree only, and not of quality. Put in the most general terms, the work of the intellect is to cause delay between stimulus and response, and under circumstances to modify the direction of the latter. We may suppose all stimulation to necessitate response, and that such response must ultimately occur with undiminished total energy. The intellect, however, is capable of delaying such response, and within limits of directing its path so that it may superficially show no relation to the stimulus of which it is the discharge. If we extend {244} the word stimulation to include the impulses arising from instinct, and grant that the delaying and deflecting influence of the intellect may be indefinitely enlarged, we have an animal in which instinct is as vigorous as in any of its primitive ancestors, but which is superficially scarcely an instinctive animal at all. Such is the case of man. His instinctive impulses are so greatly masked by the variety of response that his intellect opens to him that he has been commonly regarded until quite recent times as a practically non-instinctive creature, capable of determining by reason his conduct and even his desires. Such a conception made it almost impossible to gain any help in human psychology from the study of other animals, and scarcely less difficult to evolve a psychology which would be of the least use in foreseeing and controlling the behaviour of man.