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 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 , 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. 
