Chap. IV. MANNEK 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 be 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 of the organism than 

 on the nature of the conditions to which it has been 

 subjected. 5 



50 



Piate 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 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 subsist- 

 ence 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 the 



50 This whole subject has been discussed in chap, xxiii. vol. ii. of 

 my ' Variation of Animals and Plants under Domestication.' 



51 See the ever memorable ' Essay on the Principle of Population.' 

 by the Eev. T. Malthus, vol. i. 1826, p. 6, 517. 



K 2 



