Thus, the causes which influence birth rate are evidently very complex. In the main they are doubtless economic causes among all peoples, but there is no reason to believe that these economic causes act alone in determining birth rate, nor is there any reason to believe that the other psychological and biological causes may be in any way derived from the economic. So far as we can see, then, industrial conditions are mainly responsible for the lessened birth rate in the native white American stock. But mingled with these industrial conditions, operating as causes, are certain psychological (or moral) and biological factors that have to be considered as in the main independent. It is furthermore evident that the causes which lead to the decline and extinction of any population, whether civilized or uncivilized, are complex. All efforts to explain the extinction of peoples of antiquity, or modern nature peoples, such as the North American Indians and the Polynesians, through any single set of causes, must be looked at as unscientific. It can readily be shown that in all these cases the causes of the decline of the birth rate and the ultimate extinction of the stock are numerous and are not reducible to any single set of causes. _Causes which Influence the Death Rate._ Before we can fully understand the causes of the growth of a population, that is, of the surplus of births over deaths, we must understand something also about the things which influence the death rate as well as the things which influence the birth rate, because, let it be borne in mind, the growth of a given population (excluding immigration always) is due to the combined working of these two factors. Within certain limits the death rate is more easily controlled than the birth rate. It is very difficult for society deliberately to set about to increase the birth rate, but it is comparatively easy for it to take deliberate measures to decrease the death rate, because all individuals have a selfish interest in decreasing the death rate; but the increase of the birth rate does not appeal to the self-interest of individuals. Modern medical science, as we have seen, has done much to decrease the death rate in civilized countries, and it promises to do even more. Fifty years ago a death rate of fifty or sixty per thousand population in urban centers was not unusual, but now a death rate of thirty to forty in a thousand in the same communities is considered an intolerable disgrace, and the time will shortly come, no doubt, when even a death rate of twenty per thousand of the population will be considered disgraceful to any community. As we have already seen, the normal death rate of the most enlightened European and American communities tends to establish itself around fifteen or sixteen.