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 de- 
velopment. 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 environ- 
ment, are factors of far-reaching importance. Thrift, 
self-discipline, energy, habits of perseverance, and in- 
difference 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