534 LAWS OP MULTIPLICATION. 



plating; there are still certain changes which must prevent 

 such complete adjustment of human nature to surrounding 

 conditions, as would permit the rate of multiplication to 

 become equal to the rate of mortality. 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 which are themselves alter- 

 nately exaggerated and mitigated in the course of far longer 

 cycles; and we saw that these cause perpetual ebbings and 

 Sowings 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 resulting alterations 

 of environment. Now though the human race differs from 

 other races in having a power of artificially counteracting 

 external changes, yet there are limits to this power; and, 

 even were there no limits, the changes could not fail to work 

 their effects indirectly, if not directly. If, as is thought 

 probable, these astronomic cycles entail recurrent glacial 

 periods in each hemisphere, then parts of the Earth which are 

 at one time thickly peopled, will at another time be almost 

 deserted, and vice versa. The geologically-caused alterations 

 of climate and surface, must produce further slow re-distri- 

 butions of population; and other currents of people, to and 

 from different regions, will be necessitated by the rise of suc- 

 cessive centres of higher civilization. Consequently, man- 

 kind cannot but continue to undergo changes of environ- 

 ment, physical and moral, analogous to those which they 

 have thus far been undergoing. Such changes may eventu- 

 ally become slower and less marked; but they can never 

 cease. And if they can never cease there can never arise a 

 perfect adaptation of human nature to its conditions of exist- 

 ence. To establish that complete correspondence between 

 inner and outer actions which constitutes the highest life and 



