between a muscular frame and the strongly-pronounced supra-orbital
ridges, which are so characteristic of the lower races of man.
Besides the variations which can be grouped with more or less
probability under the foregoing heads, there is a large class of
variations which may be provisionally called spontaneous, for to our
ignorance they appear to arise without any exciting cause. It can,
however, be shewn that such variations, whether consisting of slight
individual differences, or of strongly-marked and abrupt deviations of
structure, depend much more on the constitution of the organism than on
the nature of the conditions to which it has been subjected. (56. This
whole subject has been discussed in chap. xxiii. vol. ii. of my
Variation of Animals and Plants under Domestication. )
RATE OF INCREASE.
Civilised populations have been known under favourable conditions, as
in the United States, to double their numbers in twenty-five years;
and, according to a calculation, by Euler, this might occur in a little
over twelve years. (57. See the ever memorable Essay on the Principle
of Population, by the Rev. T. Malthus, vol. i. 1826. pp. 6, 517.) At
the former rate, the present population of the United States (thirty
millions), would in 657 years cover the whole terraqueous globe so
thickly, that four men would have to stand on each square yard of
surface. The primary or fundamental check to the continued increase of
man is the difficulty of gaining subsistence, and of living in comfort.
We may infer that this is the case from what we see, for instance, in
the United States, where subsistence is easy, and there is plenty of
room. If such means were suddenly doubled in Great Britain, our number
would be quickly doubled. With civilised nations this primary check
acts chiefly by restraining marriages. The greater death-rate of
infants in the poorest classes is also very important; as well as the
greater mortality, from various diseases, of the inhabitants of crowded
and miserable houses, at all ages. The effects of severe epidemics and
wars are soon counterbalanced, and more than counterbalanced, in
nations placed under favourable conditions. Emigration also comes in
aid as a temporary check, but, with the extremely poor classes, not to
any great extent.
There is reason to suspect, as Malthus has remarked, that the
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