MANNER OF DEVELOPMENT. 53 



but checks of some kind, either periodical or constant, must 

 have kept down their numbers, even more severely than 

 with existing savages. What the precise nature of these 

 checks were we cannot say, any more than with most other 

 animals. We know that horses and cattle, which are not 

 extremely prolific animals, when first turned loose in South 

 America, increased at an enormous rate. The elephant, 

 the slowest breeder of all known animals, 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 repro- 

 duction in the wild horses and cattle of America, was at 

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

 trict became fully stocked, this same power was diminished. 

 No doubt in this case, and in all others, many checks con- 

 cur, and different checks under different circumstances; 

 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 vari- 

 able 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,* to the 

 most diversified conditions. The inhabitants of Tierra del 

 Fuego, the Cape of Good Hope, and Tasmania in the one 

 hemisphere, and of the Arctic regions in the other, must 

 have passed through many climates, and changed their 

 habits many times, before they reached their present homes, f 

 The early progenitors of man must also have tended, like 

 all other animals, to have increased beyond their means of 

 subsistence; they must, therefore, occasionally have been 

 exposed to a struggle for existence, and consequently to the 

 rigid law of natural selection. Beneficial variations of all 

 kinds will thus, either occasionally or habitually, have been 

 preserved and injurious ones eliminated. I do not refer to 

 strongly marked deviations of structure, which occur only 



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

 De<luction from Darwin's Theory," " Nature," 1869, p. 231. 

 f Latham, "Man and his Migrations," 1851, p. 135. 



