504 LAWS OF MULTIPLICATION. 



§ 375. There now remains but to inquire towards what 

 limit this progress tends. So long as the fertility of the 

 race is more than sufficient to balance the diminution by 

 deaths, population must continue to increase. So long as 

 population continues to increase, there must be pressure on 

 (lie means of subsistence. And so long as there is pressure 

 on the means of subsistence, further mental development must 

 go on, and further diminution of fertility must result Thus, 

 the change can never cease until the rate of multiplication is 

 just equal to the rate of mortality ; that is, can never cease 

 until, on the average, each pair has as many children as are 

 requisite to produce another generation of child-bearing 

 adults, equal in number to the last generation. At first 

 sight, this would seem to imply that eventually each pair will 

 rarely have more than two offspring ; but a little considera- 

 tion shows that this is a lower degree of fertility than is 

 likely ever to be reached. 



Supposing the Sun's light and heat, on which all terres- 

 trial life depends, to continue abundant, for a period long 

 enough to allow the entire evolution we are contemplating ; 

 there are still certain slow astronomic and geologic changes 

 which must prevent such complete adjustment of human nature 

 to surrounding conditions, as would permit the rate of mul- 

 tiplication to fall so low. As before pointed out (§ 148) 

 during an epoch of 21,000 years, each hemisphere goes 

 through a cycle of temperate seasons and seasons extreme in 

 their heat and cold - variations that are themselves alternately* 

 exaggerated and mitigated in the course of far longer cycles ; 

 and we saw that these caused perpetual ebbings and 

 Hovvings of species over different parts of the Earth's surface. 

 Further, by slow but inevitable geologic changes, especially 

 those of elevation and subsidence, the climate and physical 

 characters of every habitat are modified ; while old habitats 

 are destroyed and new are formed. This, too, we noted as 

 a constant cause of migrations and of consequent alterations 

 of environment. JS'ow though the human race differs from 



