"The success of an organism in leaving a numerous posterity is not measured only by the number of its surviving offspring, but also by the quality or probable success of these offspring. It is therefore a matter of importance which particular individual of those available to be their other parent. With the higher animals means of discrimination exist in the inspection of the possible mate, for in large groups the sense organs are certainly sufficiently well developed to discriminate individual differences. It is possible therefore that the emotional reactions aroused by different individuals of the opposite sex will, as in man, be not all alike, and at the least that individuals of either sex will be less easily induced to pair with some partners than with others."
The Genetical Theory of Natural Selection
Ronald Fisher (1930)