24 ‘THE SHADOW OF THE WORLD’S FUTURE 
the discoveries of methods of combating yellow-fever, 
sleeping-sickness, hookworm, malaria, filariasis, etc., 
have rendered many territories much safer for habita- 
tion—territories which formerly were so dangerous as 
to be virtually uninhabitable. To put the matter 
more briefly, advances in medicine and hygiene, by 
diminishing the risks of life, and in science and its 
applications by increasing human wealth, have already 
achieved much in helping the world to carry a con- 
siderably larger population. And it is this fact which 
appears strikingly in the astonishing collateral advance 
of population which has characterised the nineteenth 
century—yviz., an annual increase of about six-sevenths 
of 1 per cent. 
It 1s obvious that, as the world’s population develops, 
the actual population-densities in the different regions 
tend to approach the population-carrying capacities 
under the existing conditions. Since this is inevitable, 
the question of the migration of human beings is at 
once raised. Behind this, too, lies the measures of 
response to the reproductive instinct which are char- 
acteristic of different peoples. What is the multiplying 
power of various races under the various conditions 
that are possible on earth? This, it may be said, is— 
as it is in all forms of life—vastly greater than the food- 
conditions of the earth will permit to be realised. In 
Chapter I we have already seen that rates of increase 
recently experienced, could they possibly continue, 
would inevitably involve the world in difficulty. 
Adequate food-supplies are not possible. Man’s repro- 
ductive powers are held in check by what has been 
called “ the niggardliness of Nature.” How, we may 
ask, does Nature hold in check all undue increase? It 
is immediately evident that to answer this we shall 
have to make the survey of world-conditions cover 
many matters which at first, or on a superficial view, 
might have been thought quite irrelevant. In our