As in many other of these Studies, and perhaps more than in most, the task attempted in the present volume is mainly of a tentative and preliminary character. There is here little scope yet for the presentation of definite scientific results. However it may be in the physical universe, in the cosmos of science our knowledge must be nebulous before it constellates into definitely measurable shapes, and nothing is gained by attempting to anticipate the evolutionary process. Thus it is that here, for the most part, we have to content ourselves at present with the task of mapping out the field in broad and general outlines, bringing together the facts and considerations which indicate the direction in which more extended and precise results will in the future be probably found. In his famous Descent of Man, wherein he first set forth the doctrine of sexual selection, Darwin injured an essentially sound principle by introducing into it a psychological confusion whereby the physiological sensory stimuli through which sexual selection operates were regarded as equivalent to csthetic preferences. This confusion misled many, and it is only within recent years (as has been set forth in the "Analysis of the Sexual Impulse" in the previous volume of these Studies) that the investigations and criticisms of numerous workers have placed the doctrine of sexual selection on a firm basis by eliminating its hazardous csthetic element. Love springs up as a response to a number of stimuli to tumescence, the object that most adequately arouses tumescence being that which evokes love; the question of csthetic beauty, although it develops on this basis, is not itself fundamental and need not even be consciously present at all. When we look at these phenomena in their broadest biological aspects, love is only to a limited extent a response to beauty; to a greater extent beauty is simply a name for the complexus of stimuli which most adequately arouses love. If we analyze these stimuli to tumescence as they proceed from a person of the opposite sex we find that they are all appeals which must come through the channels of four senses: touch, smell, hearing, and, above all, vision. When a man or a woman experiences sexual love for one particular person from among the multitude by which he or she is surrounded, this is due to the influences of a group of stimuli coming through the channels of one or more of these senses. There has been a sexual selection conditioned by sensory stimuli. This is true even of the finer and more spiritual influences that proceed from one person to another, although, in order to grasp the phenomena adequately, it is best to insist on the more fundamental and less complex forms which they assume. In this sense sexual selection is no longer a hypothesis concerning the truth of which it is possible to dispute; it is a self-evident fact. The difficulty is not as to its existence, but as to the methods by which it may be most precisely measured. It is fundamentally a psychological process, and should be approached from the psychological side. This is the reason for dealing with it here. Obscure as the psychological aspects of sexual selection still remain, they are full of fascination, for they reveal to us the more intimate sides of human evolution, of the process whereby man is molded into the shapes we know.