Now among the lower animals, this union is irrational in character; it exists merely for the purpose of procreation, and lasts only so long as the parents are occupied in producing their brood. In tame animals, on the other hand, and those which possess a greater share of intelligence, it has assumed a more complex form; for in their case we see more examples of mutual help, goodwill, and co-operation.

It is, however, in the human species that this complexity is most marked; since the co-operation between woman and man aims not merely at existence, but at a happy existence. Nor do mankind beget children merely to pay the service they owe to Nature, but also that they may themselves receive a benefit; for the toil they undergo while they are strong and their offspring is still weak is repaid by that offspring when it in turn is grown strong and the parents by reason of age are weak.