but it seems to me that this regard for their own welfare or that of others, however we may phrase it, is the only guiding-principle of conduct, in the light of which men can reconsider and review their rules.  Unless they follow the mere blind impulses of feeling (in which case they do not follow rules at all, but simply act irrationally), or else observe implicitly the maxims of conduct which they find prevalent around them, they must, and can only, ask the question whether it is possible to alter their conduct for the better, that is to say, whether they can better promote their own welfare or that of others by some modification of their actions.  Take the case of Slavery.  There was a time when savage or barbaric tribes, moved by a regard to their own interests, and also, we may trust, touched by some compassion for their victims, began to substitute, for the wholesale butchery of their enemies defeated in war, the practice of retaining some or all of them for the purposes of domestic or agrarian service.  Again, there came a time when, viewed by the side of other forms of service which had meanwhile come into existence, slavery, with its various incidents, began to shock the philanthropic sentiments of the more civilized races of mankind, while the question also began to be raised whether slave-labour was not economically at a disadvantage, when compared with free labour, and the result of these combined considerations, often aided by a strong and enthusiastic outburst of popular feeling, has been the total disappearance of slavery amongst civilized, and its almost total disappearance even amongst barbaric or semi-civilized races.  Take, too, the revolting practice, common among many savage tribes, past and present, of killing and eating aged parents or other infirm members of the tribe, when engaged in war.  This practice which, at first sight, seems so utterly unnatural, was doubtless dictated, in part at least, by the desire to save their victims from the worse fate of being tortured and mutilated by their enemies.  Subsequently, in the history of some of these tribes, there has come a time when it has been discovered that a more humane mode of attaining the same object is to build strong places and leave the feebler folk at home.  If we follow the varying marriage customs of savage or barbaric tribes, we shall find, in the same way, that they have always been originally framed on reasons of convenience, and that, when they have been changed, it has been because different views of well-being, including the needs of purity, closer attachment, increased care of children, and the like, have begun to prevail.  In all these examples, which might be multiplied to any extent, it is plain that changes of conduct are moulded and determined by changes of opinion as to what is best and most suitable for the circumstances of the individual, the family, the tribe, or whatever the social aggregate may be.  And I may venture to affirm that, wherever any change of moral conduct takes place, unless it