126 THE DESCENT OF MAN. [Part 1 



hair, color and constitution, are more or less correlated. 4 ' 

 Prof. Schaaffhausen first drew attention to the relation 

 apparently existing between a muscular frame and strong- 

 ly-pronounced supra-orbital ridges, which are so char- 

 acteristic 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 they appear, owyig to our igno- 

 rance, to arise without any exciting cause. It can, how- 

 ever, be shown 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. 1 



60 



Rate of Increase. — Civilized populations have been 

 known under favorable conditions, as in the United States, 

 to double their number in twenty-five years; and, ac- 

 cording to a calculation by Euler, this might occur in a 

 little over twelve years." At the former rate the present 

 population of the United States, narhely, thirty millions, 

 would in 657 years cover the whole terraqueous gldbe 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 



49 The authorities for these several statements are given in my 

 4 Variation of Animals under Domestication,' vol. ii. pp. 320-335. 



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

 * Variation of Animals and Plants under Domestication.' 



61 See the ever-memorable ' Essay on the Principle of Population,' 

 by the Rev. T. Malthus, vol. i. 182G, pp. 6, 51V. 



