HOW POPULATION INCREASES 59 
forming to the principle just stated in (iv) of the factors 
above. Touching the matters referred to in (v) and 
(vi), doubtless better personal hygiene will tend to 
make a population more efficient, and this also will 
indirectly tend to bring about more favourable 
economic conditions. In this connection may be 
mentioned the recent uprising of the idea of “con- 
structive birth-control,” advocated by Dr Marie C. 
Stopes, Mrs Sanger, and others, and in many countries 
in Europe. The aim of this is to avoid reproduction, 
when undesired, with a view to ensuring that the 
conditions surrounding reproduction shall be more 
adequately considered, and practically attended to. 
Reproduction, under better conditions than those at 
present prevailing, will also tend to increase human 
efficiency, and will improve the human race probably in 
every way. On the other hand, however, any widen- 
ing of the knowledge of contraceptive technique will 
probably tend to somewhat limit reproduction, at 
least of the worse kind. Statistical evidence does not 
yet exist as to the ultimate effect of this movement, 
hence at present its consequences have to be inferred 
from general considerations. The importance of 
attention to constructive birth-control is obvious from 
such evidence as is given in Dr F. G. Crookshank’s 
The Mongol in our Midst, 1925, and also in various 
studies of the consequences of reproduction from 
degenerate human beings. 
A review of the surface-conditions of various 
countries shows that many could carry probably much 
greater populations than they do at the present time, 
and some could certainly carry very much greater 
populations, ¢.g., Russia-in-Asia, the northern part of 
Asia generally, Persia, Irak, etc., a considerable part of 
Africa, Canada, Central and South America, Brazil, 
the Argentine, etc, Australia and New Guinea. 
Recent advances in hygiene for cold and tropical