As if to make assurance doubly sure, natural selection here steps in to further the process. To prove this with all the rigidity of demonstration desirable is in the present state of erotics beyond our power. Until our family trees give us something more than mere skeletons of dead branches, we must perforce continue ignorant of the science of grafts. For the nonce we must be content to generalize from our own premises, only rising above them sufficiently to get a bird's-eye view of our neighbor's estates. Such a survey has at least one advantage: the whole field of view appears perfectly plain. Surveying the subject, then, from this ego-altruistic position, we can perceive why matrimony, as we practise it, should result in increasing the personality of our race: for the reason namely that psychical similarity determines the selection. At first sight, indeed, such a natural affinity would seem to have little or nothing to do with marriage. As far as outsiders are capable of judging, unlikes appear to fancy one another quite as gratuitously as do likes. Connubial couples are often anything but twin souls. Yet our own dual use of the word "like" bears historic witness to the contrary. For in this expression we have a record from early Gothic times that men liked others for being like themselves. Since then, our feelings have not changed materially, although our mode of showing them is slightly less intense. In those simple days stranger and enemy were synonymous terms, and their objects were received in a corresponding spirit. In our present refined civilization we hurl epithets instead of spears, and content ourselves with branding as heterodox the opinions of another which do not happen to coincide with our own. The instinct of self-development naturally begets this self-sided view. We insensibly find those persons congenial whose ideas resemble ours, and gravitate to them, as leaves on a pond do to one another, nearer and nearer till they touch. Is it likely, then, that in the most important case of all the rule should suddenly cease to hold? Is it to be presumed that even Socrates chose Xantippe for her remarkable contrariety to himself? Mere physical attraction is another matter. Corporeally considered, men not infrequently fall in love with their opposites, the phenomenally tall with the painfully short, the unnecessarily stout with the distressingly slender. But even such inartistic juxtapositions are much less common than we are apt at times to think. For it must never be forgotten that the exceptional character of the phenomena renders them conspicuous, the customary more consorted combinations failing to excite attention.