Chap. IV. MANNER OF DEVELOPMENT. 135 



offspring. There would have been no prudential re- 

 straint from marriage, and. the sexes would have freely 

 united at an early age. Hence the progenitors of 

 man would have tended to increase rapidly, 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 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 reproduction 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 diminished. 

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

 concur, and different checks under different circum- 

 stances ; periodical dearths, depending on unfavourable 

 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 boclv 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, 56 to the most diversified con- 



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

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



