68 THE DESCENT OF MAN 



taneous, for to our ignorance they appear to arise without 

 any exciting cause. It can, however, be shown that such 

 variations, whether consisting of slight individual differ- 

 ences, 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. " 



Mate of Increase. — Civilized populations have been known 

 under favorable conditions, as in the United States, to double 

 their numbers in twenty- five years; and, according to a cal- 

 culation by Euler, this might occur in a little over twelve 

 years." 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 civilized 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 counter- 

 balanced, and more than counterbalanced, in nations placed 

 under favorable 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 



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

 "Variation of Animals and Plants under Domestication." 



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

 Eev. T. Malthus, vol, i,, 1826, pp. 6, 517. 



