Fison and Howitt[234] give other examples of marriage by exchange, and I have already given a description of the custom of _Tualcha mura_, the _regular_ method of obtaining a wife among the central Australians, by means of which a man secures a wife for his son by making an arrangement with some other man with regard to the latter's daughter. From the evidence given first of all I think we must conclude that early man was inclined to appropriate whatever women came in his way. In this regard we have a condition resembling that among the higher animals, where the more vigorous males try to monopolize the females. We may assume also that the women first appropriated were those born in the group--that is, in the immediate family--as being more proximate and not already possessed by others. In this regard also the condition resembled that among the higher gregarious animals; and in so far as the control of the women by the men of the group is concerned the condition remains unchanged. But the men have ceased to marry the women of their immediate families, and the problem of exogamy is to determine why men living with women and controlling them should cease to marry them. In other papers I have pointed out that the interest of man is not held nor the emotions aroused when the objects of attention have grown so familiar in consciousness that the problematical and elusive elements disappear;[235] and I have also alluded to the laws of sexual life, that an excited condition of the nervous system is a necessary preparation to pairing.[236] And just here we must recognize the fact that monogamy is a habit acquired by the race, not because it has answered more completely to the organic interest of the individual, but because it has more completely served social needs, particularly by assuring to the woman and her children the undivided interest and providence of the man. But in early times the law of natural selection, not the law of choice, operated to preserve the groups in which a monogamous or quasi-monogamous tendency showed itself (since the children in these cases were better trained and nourished), and in historical times and among ourselves all of the machinery of church and state has been set in motion in favor of the system. In point of fact, the members of civilized societies at the present time have become so refined and have so far accepted ethical standards that monogamy is the system actually favored on sentimental grounds as well as on grounds of expediency by a large proportion of any civilized population. On the other hand, speaking from the biological standpoint, monogamy does not, as a rule, answer to the conditions of highest stimulation, since here the problematical and elusive elements disappear to some extent, and the object of attention has grown so familiar in consciousnes that the emotional reactions are qualified. This is the fundamental explanation of the fact that married men and