marriages.  In narrow communities, where breeding in-and-in becomes almost inevitable, natural selection has similarly to exert itself upon a crowd of cretins and other hapless incapables.  But in wide and open champaign countries, where individual choice has free room for exercise, men and women as a rule (if not constrained by parents and moralists) marry for love, and marry on the whole their natural complements.  They prefer outsiders, fresh blood, somebody who comes from beyond the community, to the people of their own immediate surroundings.  In many men the dislike to marrying among the folk with whom they have been brought up amounts almost to a positive instinct; they feel it as impossible to fall in love with a fellow-townswoman as to fall in love with their own first cousins.  Among exogamous tribes such an instinct (aided, of course, by other extraneous causes) has hardened into custom; and there is reason to believe (from the universal traces among the higher civilisations of marriage by capture) that all the leading races of the world are ultimately derived from exogamous ancestors, possessing this healthy and excellent sentiment.

In minor matters, it is of course universally admitted that short men, as a rule, prefer tall women, while tall men admire little women.  Dark pairs by preference with fair; the commonplace often runs after the original.  People have long noticed that this attraction towards one’s opposite tends to keep true the standard of the race; they have not, perhaps, so generally observed that it also indicates roughly the existence in either individual of a desire for its own natural complement.  It is difficult here to give definite examples, but everybody knows how, in the subtle psychology of Falling in Love, there are involved innumerable minor elements, physical and mental, which strike us exactly because of their absolute adaptation to form with ourselves an adequate union.  Of course we do not definitely seek out and discover such qualities; instinct works far more intuitively than that; but we find at last, by subsequent observation, how true and how trustworthy were its immediate indications.  That is to say, those men do so who were wise enough or fortunate enough to follow the earliest promptings of their own hearts, and not to be ashamed of that divinest and deepest of human intuitions, love at first sight.

How very subtle this intuition is, we can only guess in part by the apparent capriciousness and incomprehensibility of its occasional action.  We know that some men and women fall in love easily, while others are only moved to love by some very special and singular combination of peculiarities.  We know that one man is readily stirred by every pretty face he sees, while another man can only be roused by intellectual qualities or by moral beauty.  We know that sometimes we meet people possessing every virtue and grace under heaven, and yet for some unknown and incomprehensible reason