130 THE DESCENT OF MAN. [Paet L 



tended to increase rapidly, but checks of some kind, either 

 periodical or constant, must have kept down their num- 

 bers, even more severely than with existing savages. 

 What the precise nature of these checks may have been, 

 we cannot say, any more than with most other animals. 

 We know that horses and cattle, which are not highly 

 prolific animals, when first turned loose in South America, 

 increased at an enormous rate. The slowest breeder of 

 all known animals, namely the elephant, would in a few 

 thousand years stock the whole world. The increase of 

 every species of monkey must be checked by some means ; 

 but not, as Brehm remarks, by the attacks of beasts of 

 prey. No one will assume that the actual power of re- 

 production in the wild horses and cattle of America was 

 at first in any sensible degree increased ; or that, as each 

 district became fully stocked, this same power was dimin- 

 ished. JSTo doubt in this case, and in all others, many 

 checks concur, and different checks under different circum- 

 stances ; periodical dearths, depending on unfavorable 

 seasons, being probably the most important of all. So it 

 will have been with the early progenitors of man. 



Natural Selection. — We have now seen that man is 

 variable in body and mind ; and that the variations are 

 induced, either directly or indirectly, by the same general 

 causes, and obey the same general laws, as with the lower 

 animals. Man has spread widely over the face of the 

 earth, and must have been exposed, during his incessant 

 migrations, 66 to the most diversified conditions. The in- 

 habitants of Tierra del Fnego, the Cape of Good Hope, 

 and Tasmania in the one hemisphere, and of the Arctic 

 regions in the other, must have j)assed through many cli- 

 mates and changed their habits many times, before they 



68 See some good remarks to this effect by "W. Stanley Jevons, " A 

 Deduction from Darwin's Theory," 'Nature,' 1869, p. 231. 



