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