50 THE DESCENT OF MAN. 



Rate of Increase. Civilized populations have been 

 known under favorable conditions, as in the United States, 

 to double their numbers in twenty-five years ; and, accord- 

 ing to a calculation, by Euler, this might occur in a little 

 over twelve years. * At the former rate, the present popu- 

 lation 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 sur- 

 face. 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 miser- 

 able houses at all ages. The effects of severe epidemics 

 and wars are soon counterbalanced, and more than counter- 

 balanced 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 

 the reproductive power is actually less in barbarous, 

 than in civilized races. We know nothing positively 

 on this head, for with savages no census has been taken ; 

 but from the concurrent testimony of missionaries, and of 

 others who have long resided with such people, it appears 

 that their families are usually small, and large ones rare. 

 This may be partly accounted for, as it is believed, by the 

 women suckling their infants during a long time ; but it 

 is highly probable that savages, who often suffer much 

 hardship, and who do not obtain so much nutritious food 

 as civilized men, would be actually less prolific. I have 

 shown in a former work, f that all our domesticated quad- 

 rupeds and birds, and all our cultivated plants, are more 

 fertile than the corresponding species in a state of nature. 



* See the ever memorable "Essay on the Principle of Population," 

 by the Rev. T. Malthus, vol. i, 1826, pp. 6, 517. 



f " Variation of Animals and Plants under Domestication," vol. ii, 

 pp. 111-113, 163. 



