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as for example Belgium with 673, Netherlands with 499, etc. But it 
must not be forgotten that these countries are far from self-supporting in 
respect of physical means of subsistence. They are economically self- 
supporting, which is a very different thing, because by their industrial 
development at home and in their colonies they produce money enough 
to buy physical means of subsistence from less densely populated portions 
of the world. We can, of course, do the same thing, provided that by 
the time our population gets so dense as to make it necessary there still 
remain portions of the globe where food, clothing material, and fuel are 
produced in excess of the needs of their home population. But in this, 
and in any other scientific discussion of population, it is necessary to 
limit sharply the area one is to talk about. This paper deals with popula- 
tion, and by direct implication the production of physical means of sub- 
sistence, within the present area of continental United States. 
Now 197,000,000 people will require, on the basis of our present food 
habits,io about 260,000,000,000,000 calories per annum. The United 
States, during the seven years 1911-1918, produced as an annual average, 
in the form of human food, both primary and secondary (i.e., broadly 
vegetable and animal), only 137,163,606,000,000 calories per year.^^ So 
that unless our food habits radically change, and a man is able to do with 
less than 3000 to 3500 calories per day, or unless our agricultural pro- 
duction radically increases, it will be necessary when our modest figure 
for the asymptotic population is reached, to import nearly or quite one- 
half of the calories necessary for that population. It seems improbable 
that the population will go on increasing at any very rapid rate after such 
a condition is reached. And is it at all reasonable to suppose that at such 
time, with all the competition for means of subsistence which the already 
densely populated countries of Europe will be putting up, there can be 
found any portion of the globe producing food in excess of its own needs to 
an extent to make it possible for us to find the calories we shall need to 
import? 
Altogether, we believe it will be the part of wisdom for anyone disposed 
to criticise out asymptotic value of a hundred and ninety-seven and a 
quarter millions because it is thought too small, to look further into all 
the relevant facts. 
Ill 
With the above numerical results in hand it is desirable to discuss a 
little further the general theory of population growth set forth in the 
preceding section. At the outstart let it be said that we are convinced 
that equation (ix) represents no more than a first approximation to a 
true law of population growth. There are several characteristics of this 
curve which are too rigid and inelastic to meet the requirements of such a 
law. In (ix) the point of inflection must of necessity lie exactly half-way 
between the two asymptotes. Furthermore the half of the curve lying 
