MANNKK OF DEVELOPMENT. 43 



ing of slight indwidiial differences, or of strongly-marked and 

 abrupt deviations of structure, depend much more on '.he consti- 

 tution of the organism than on the nature of the cowditions to 

 which it has been subjected."" 



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, according 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 (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 con- 

 tinued 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 restrain- 

 ing 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 counterbalanced, 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 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 be- 

 lieved, 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,"' that all our domesticated quadrupeds and birds, 

 and all our cultivated plants, are more fertile than the cor- 

 responding species in a state of nature. It is no valid objection to 



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

 'Variation of Animals and Plants under Dcmestioation.' 



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

 the Rev. T. Malthus, vol. i. 1826, p. 6, 517. ■ 



=* 'Variation of Animals and Plants under Domestication,' vol. ii. 

 pp. 111-113, 163. 



