But it is especially among birds and mammals that we find forms of union or association very similar to marriage and the family in the human species. Nothing is more natural; for anatomical and physiological analogy must of necessity lead in its train the analogy of sociology. There is no more uniformity either amongst mammals than amongst men; the needs, the habitat, and the necessities of existence dominate everything, and in order to secure adjustment to these, recourse is had to various processes. Like men, birds live sometimes in promiscuity, and sometimes in monogamy or polygamy; the familial instinct is also very unequally developed among them. Sometimes even we find their conjugal customs modified by the kind of life they lead. Thus the wild duck, which is strictly monogamous in a wild state, becomes very polygamous when domesticated, and it is the same with the guinea fowl. Civilisation depraves these birds, as it does some men. As may be supposed, it is generally the animals living in troops who are degraded most easily by habitual promiscuity. But this is not always the case; the character of the animal, his mode of life, and the degree of morality previously acquired, determine his manner of acting. It is probable also that certain animals, living in troops during the breeding season, have formerly been less sociable than at present, for they leave the troop and retire apart in couples as soon as they have paired. Social life is burdensome to them. It is especially interesting to study the various modes of conjugal and familial association amongst birds. This may easily be inferred from the ardour, the variety, and the delicacy they bring to their amours; the moral level among them, to borrow a human expression, is very diverse, according to the species. There are some birds absolutely fickle and even debauched--as, for example, the little American starling (_Icterus pecoris_), which changes its female from day to day; that is to say, it is in the lowest stage of sexual union, a debauched promiscuity, which we only exceptionally find in some hardly civilised human societies.[38] The starling, nevertheless, is not ferocious, like the asturides, to whom, according to Brehm, love seems unknown, and amongst whom the female devours her male, the father and the mother feast on their own young, and the latter, when full grown, willingly eat their parents. These ferocious habits denote a very feeble moral development. But if we may believe a French missionary, Mgr. Farand, bishop of the Mackenzie territory, similar customs still prevail among the Redskins of the extreme north.[39] We shall not, therefore, be too much scandalised at the birds. These cases of moral grossness are, besides, rare enough with them.