348 ENVIRONMENT AMONG MEN 



says Mr. McDougall, * that defect of the functions of this organ 

 (thyroid) may reduce any one of us to a state of mental apathy 

 bordering upon idiocy, and that its excessive activity produces 

 the opposite effect and may throw the mind into an over-excitable 

 condition verging upon maniacal excitement. Again we know 

 that certain diseases tend to produce specific changes of tempera- 

 ment, that phthisis often gives it a bright and hopeful turn, 

 diabetes a dissatisfied and cantankerous turn. It is clear that 

 in some such cases of profound alteration of temperament by 

 bodily disorder the effects are produced by means of the chemical 

 products of metabolism, which, boing thrown out of the diseased 

 tissues into the blood and reaching the nervous system by way of 

 the blood-stream, chemically modify its processes. It is probable 

 that every organ in the body exerts in this way some influence 

 upon our mental life, and that temperament is in large measure 

 the balance or resultant of all these many contributory influences.' 1 

 Thus in addition to the more obvious direct effects of disease 

 upon mental and physical characters, there are the more subtle 

 and profound effects upon temperament, and in our general 

 summing-up we shall find reason to attribute no small importance 

 to temperament in its influence upon progress. It thus becomes 

 of importance to observe that in particular regions of the world 

 certain chronic and non-lethal diseases are very common. Clearly 

 the result of disease upon temperament is greatest when it takes 

 the form, not of kill or cure which is on the whole the case in 

 temperate climates but of chronic non-lethal maladies which 

 are common in tropical climates, especially in Africa. The hook- 

 worm disease is an African disease which has been introduced 

 into America. The hook-worm (Ancylostomum duodenale) is an 

 internal parasite which attaches itself to the lining of the intestine 

 and causes bleeding and anaemia. Death may ensue, but the 

 patient usually lives a long time. It is a most debilitating disease ; 

 it is said to affect some 50 per cent, of the inhabitants of some 

 tropical and sub-tropical countries. Even as a mild infection 

 it is the cause of much invalidity and heavy economic loss. The 

 laziness and degeneracy of the ' poor white trash ', who are said 

 to belong to. the purest -blooded English stock in the United 

 States, are due not so much to the environment 2 and heredity 



1 McDougall, Social Psychology, p. 118. 2 Here environment clearly means 



social or traditional environment. 



