Whenever such statements as these are made it is always retorted that love laughs at science, and that the winds of passion blow where they list.[155] That, however, is by no means altogether true, and in any case it is far from covering the whole of the ground. It is hard to fight against human nature, but human nature itself is opposed to indiscriminate choice of mates. It is not true that any one tends to love anybody, and that mutual attraction is entirely a matter of chance. The investigations which have lately been carried out show that there are certain definite tendencies in this matter, that certain kinds of people tend to be attracted to certain kinds, especially that like are attracted to like rather than unlike to unlike, and that, again, while some kinds of people tend to be married with special frequency other kinds tend to be left unmarried.[156] Sexual selection, even when left to random influences, is still not left to chance; it follows definite and ascertainable laws. In that way the play of love, however free it may appear, is really limited in a number of directions. People do not tend to fall in love with those who are in racial respects a contrast to themselves; they do not tend to fall in love with foreigners; they do not tend to be attracted to the ugly, the diseased, the deformed. All these things may happen, but they are the exception and not the rule. These limitations to the roving impulses of love, while very real, to some extent vary at different periods in accordance with the ideals which happen to be fashionable. In more remote ages they have been still more profoundly modified by religious and social ideas; polygamy and polyandry, the custom of marrying only inside one's own caste, or only outside it, all these various and contradictory plans have been easily accepted at some place and some time, and have offered no more conscious obstacle to the free play of love than among ourselves is offered by the prohibition against marriage between near relations. Those simple-minded people who talk about the blind and irresistible force of passion are themselves blind to very ordinary psychological facts. Passion--when it occurs--requires in normal persons cumulative and prolonged forces to impart to it full momentum.[157] In its early stages it is under the control of many influences, including influences of reason. If it were not so there could be no sexual selection, nor any social organization.[158]