Those persons who are attracted to both sexes are now usually termed "bisexual," a more convenient term than "psycho-sexual hermaphrodite," which was formerly used.

There remains the normal person, who is "heterosexual." Before approaching the study of sexual inversion in cases which we may investigate with some degree of scientific accuracy, there is interest in glancing briefly at the phenomena as they appear before us, as yet scarcely or at all differentiated, among animals, among various human races, and at various periods.
Among animals in a domesticated or confined state it is easy to find evidence of homosexual attraction, due merely to the absence of the other sex.[2] This was known to the ancients; the Egyptians regarded two male partridges as the symbol of homosexuality, and Aristotle noted that two female pigeons would cover each other if no male was at hand.

Buffon observed many examples, especially among birds.

He found that, if male or female birds of various species--such as partridges, fowls, and doves--were shut up together, they would soon begin to have sexual relations among themselves, the males sooner and more frequently than the females.

More recently Sainte-Claire Deville observed that dogs, rams, and bulls, when isolated, first became restless and dangerous, and then acquired a permanent state of sexual excitement, not obeying the laws of heat, and leading them to attempts to couple together; the presence of the opposite sex at once restored them to normal conditions.[3] Bombarda of Lisbon states that in Portugal it is well known that in every herd of bulls there is nearly always one bull who is ready to lend himself to the perverted whims of his companions.[4] It may easily be observed how a cow in heat exerts an exciting influence on other cows, impelling them to attempt to play the bull's part.