HUMAN POPULATION IN THE FUTURE. 535 



greatest power of self-preservation, there must be a prolonged 

 converse between the organism and circumstances which re- 

 main the same. If the external relations are being altered 

 while the internal relations are being adjusted to them, the 

 adjustment can never become exact. And in the absence of 

 exact adjustment, there cannot exist that theoretically-highest 

 power of self-preservation with which there would co-exist 

 the theoretically-lowest power of race-production. 



Hence though the number of premature deaths may ulti- 

 mately become very small, it can never become so small as 

 to allow the average number of offspring from each pair to 

 fall so low as two. Some average number between two and 

 three may be inferred as the limit — a number, however, which 

 is not likely to be quite constant, but may be expected at 

 one time to increase somewhat and afterwards to decrease 

 somewhat, according as variations in physical and social con- 

 ditions lower or raise the cost of self-preservation. 



To this qualification must be added a further qualifica- 

 tion. The foregoing argument tacitly assumes that the 

 causes described will continuously operate on all mankind; 

 whereas a survey of the facts makes it clear that some parts 

 only of the Earth's surface are capable of bearing high 

 types of civilization, and consequently high types of Man. 

 There must remain hereafter, as there are now, considerable 

 parts of its surface which can support only groups of nomads, 

 or other groups obliged by their habitats to lead simple 

 and inferior kinds of life. /Only by subjection to the disci- 

 pline we have been contemplating can there be produced the 

 fully-developed Man; and evidently in many parts of the 

 world this discipline will continue to be eluded. \ Not only 

 must we conclude that the varieties of our race now liv- 

 ing in desert regions and arctic climates will continue here- 

 after to do so, but we may conclude that always, as now, a 

 certain proportion of men who are born in civilized societies, 

 impatient of the stress which pressure of population puts 

 on them, will escape into unoccupied or sparsely-peopled 



