In his "Origin of the Family," Frederick Engels says:
"The development of the family is founded on the continual contraction of the circle, originally comprising the whole tribe, within which marital intercourse between both sexes was general. By the continual exclusion, first of near, then of ever remoter relatives, including finally even those who were simply related legally, all group marriage becomes practically impossible. At last only one couple, temporarily and loosely united, remains ... even from this we may infer how little the sexual love of the individual in the modern sense of the word had to do with the origin of monogamy."
Any casual student of sociology can prove that marriage and the family have not always been what they are today. Lewis J. Morgan, in his well-known work, "Ancient Society," says:
"When the fact is accepted that the family has passed through four successive forms, and is now in a fifth, the question at once arises whether this form can be permanent in the future. The only answer that can be given is that it must advance as society advances, and change as society changes, even as it has done in the past. It is the creature of the social system and will reflect its culture."
Engels says:
"We have three main forms of the family, corresponding in general to the three main stages of human development. For savagery group marriage, for barbarism the pairing family, for civilization, monogamy supplemented by adultery and prostitution."
THE PAIRING FAMILY
"A certain pairing for a longer or shorter term took place even during the group marriage or still earlier. A man had his principal wife among other women, and he was to her the principal husband among others.... Such a habitual pairing would gain ground the more the gens developed and the more numerous the classes of "brothers" and "sisters" became who were not permitted to marry one another....
"By this increasing complication of marriage restrictions, group marriage became more and more impossible; it was displaced by the pairing family.