[Sidenote: 2. The development of morality: (_a_) historical psychology.] The way in which the theory of evolution is applied to ethical psychology is easy enough to understand in principle, though complex and obscure in many of its details. We have only to postulate that mental as well as bodily traits admit of modification, and that modifications once produced can be transmitted to descendants,[110] and it at once follows that sentiments and ideas leading to actions which promote life will be encouraged and developed by natural selection. Thus parental and filial feelings, once originated, may have been developed through those families and tribes in which they were strongest, presenting a more united, and therefore stronger, front against hostile influences. The feelings of tribal sympathy and patriotism, too, may have had a similar history. Those races in which they were strongest would, other things being equal, obtain the mastery over and exterminate other races in which they were relatively weak. The compactness of the community would even be promoted by that fear of the political and of the religious control in which the feeling of obligation is said to have had its root. In general, benevolence and sympathy amongst a people give it a solidarity from which it derives a stronger position, so that in turn the benevolent and sympathetic feelings gain free scope to develop and expand. [Sidenote: Its difficulties: the origin of new feelings,] [Sidenote: and of the moral consciousness;]