ENVIRONMENT AMONG MEN 353 



It is probable that physical predispositions are on the whole 

 more susceptible to such changes as occur than are mental 

 predispositions. There is no reason, for instance, to think that 

 the normal variation of any factor which influences mental 

 predispositions produces as much effect as does use upon muscular 

 development. But it is also further clear that, though we must, 

 keeping aside for the moment questions connected with disease 

 and with temperament, think of such modifications as occur as 

 not of much importance, nevertheless these modifications are of 

 greater importance than among species in a state of nature closely 

 related to man. Consider for a moment muscular development. 

 Side by side in the same street may live a man whose daily work 

 is wholly sedentary and whose muscles are in consequence 

 undeveloped, and a man who works in a mine or engineering 

 shop and develops his muscles accordingly. Nowhere among 

 vertebrate animals in a state of nature are such differences 

 found. 



Though but little is definitely known on the subject, it seems 

 that temperament is more susceptible to changes in the environ- 

 ment than are other mental or physical characters. Changes of 

 climate, for example, appear to produce more marked changes 

 in temperament than in any other character. Temperament is 

 certainly very susceptible to the influence of disease, which may 

 also profoundly affect all characters both mental and physical. 

 It is probably in its effects upon temperament that disease has 

 its chief importance. We may even have to recognize in the 

 effects produced by certain tropical diseases a serious hindrance 

 to progress in tropical countries. It is difficult, however, to 

 disentangle from the effects of disease in this sense the effects 

 which it produces in the sense awaiting consideration in later 

 chapters. Clearly the wide prevalence of disease, where such is 

 the case, must form an important factor in the surroundings 

 which the mind has for the subject-matter of its activities — 

 but on this and on alhed subjects there will be more to say later. 



We have already in what has just been said referred to one 

 kind of, modification in its effect upon progress. Upon this 

 subject in general it may in the first place again be emphasized 

 that, so far as is at present known, there are no grounds for 

 believing that modifications of the kind considered in this chapter 

 give rise to mutations. From this it follows that the results 



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