Chap. IV. 
MANNER OF DEVELOPMENT. 
131 
tion apparently existing between a muscular frame and 
strongly-pronounced supra-orbital ridges, which are so 
characteristic of the lower races of man. 
Besides the variations which can bo grouped with 
more or less probability under the foregoing heads, 
there is a large class of variations which may be provi- 
sionally called spontaneous, for they appear, owing to our 
ignorance, to arise without any exciting cause. It can, 
however, be shewn that such variations, whether con- 
sisting of slight individual differences, or of strongly- 
marked and abrupt deviations of structure, depend 
much more on the constitution ol the organism than 
on the nature of the conditions to which it has been 
subjected. 50 
Bate of Increase. — Civilised populations have been 
known under favourable conditions, as in the United 
States, to double their number in twenty-five years; 
and according to a calculation by Euler, this might 
occur in a little over twelve years. 51 At the former rate 
the present population of the United States, namely, 
thirty millions, would iu 657 years cover the whole 
terraqueous globe so thickly, that four men would have 
to stand on each square yard of surface. The primary 
°r 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 
"'e see, for instance, in the United States, where subsist- 
ence is easy and there is plenty of room. If such means 
"ere suddenly doubled in Great Britain, our number 
"'Quid be quickly doubled. With civilised nations the 
50 This whole subject has been discussed in chap, xsiii. vol. ii. of 
®y ‘ Variation of Animals and Plants under Domestication.’ 
51 See the ever memorable ‘ Essay on the Principle of Population,’ 
b y the Kev. T. Malthus, vol. i. 1826, p. G, 517. 
K 2 
