To answer this we shall have to go back a little to first principles, and consider, however briefly, certain facts as to the march of evolution. Evolution is essentially progressive. It proceeds on the whole in a certain direction, and that direction is on the whole towards a realization of what seems to us to have positive value. The direction, however, as a matter of fact, is most striking when we consider the maximum level attained, much less so when we consider the average, not at all when we look at the minimum. The method or mechanism of progress may differ in different types, and it does differ in man from that which is found in other mammals. In most higher animals progress is brought about chiefly by natural selection operating upon individuals, although in a few forms selection operates chiefly upon groups of communities: in both cases the changes in the inherited constitution of the species are the important changes. In man, however, in all except the very early stages of his development, changes in inherited constitution have been small and unimportant, and the chief changes of evolutionary significance have been those in tradition; selection among individuals has been of relatively little importance, and selection has fallen mainly upon groups and, to an ever-increasing extent, upon their ideas and traditions. In spite of differences in method as between different types of organism, the tendency has been in the same direction--towards a possibility of greater control, greater independence, greater complexity, and greater regulation or harmony. Looked at from the evolutionary point of view, the moving, dynamic point of view, we have to think of human sex-psychology in yet another way. So far we have been treating it as what it is; now we must think of what it may become. The general rule in evolution--the natural and obvious rule--is that acquisitions are not thrown away when change occurs, but built upon, utilized for some new function. The endostyle of the lowest chordates, part of a very primitive type of feeding mechanism, was converted, when they changed their mode of life, into the thyroid gland: the parathyroids develop from the remains of the gill-apparatus when gills are discarded for lungs: the secondary sexual differences which originate as accidental consequence of the primary difference between the sexes are, over and over again, elaborated into special characters employed in courtship.