THE MIGRATION OF POPULATIONS 81
energy, and the possibilities for him of the mineral,
vegetable and animal wealth, which potentially is at
his command. These are what may be called the
environmental factors.
There are also what may be called the human or
personal factors, and these are also of the first order of
importance. In turning his resources to account
Man needs knowledge, inventiveness, and that insight
which leads to discovery. He needs also strength of
purpose, courage, the capacity to create and to regulate
the social organism, and the ethical qualities by means
of which all human relationships are ameliorated, and
which facilitate both his economic and his social development.
 All these things affect his scatter upon the
earth’s surface.
To be most effective in the purposes of life, he must
perfect both his physical and his mental organism;
in other words a good physical and a good mental
hygiene are desiderata which profoundly affect his
well-being and the security of his territorial occupation.
Clearly he needs also to create and to develop such
a scheme of economic and of social relations with his
fellows, as will best conserve his energies and render
them most fruitful and beneficent. For, by reducing
all clashing of interest to a minimum, it becomes
possible for his productivity to become a maximum.
Thus the attainment of individual, communal, national
and international justice, the systematic spread of
acquired knowledge, the co-ordination of all effort to
secure the most complete acquaintance with his environment,
 are factors of far-reaching importance. Thrift,
self-discipline, energy, habits of perseverance, and indifference
 to luxury are all conducive to increase of
population-density. ‘Thus, as previously indicated, the
ethical elements of Man’s “make-up” are essential to
his well-being. This is a difficulty when we take into
account something more than mere numbers in regard