32 THE CAUSATION OF DISEASE. 



same for all ; but as we advance from barbaric to civilized 

 peoples, the E grows more and more complex and diverse; 

 not only, that is to say, is it more elaborate and comprehen- 

 sive for each individual, but, owing to the division cf 

 labour, the E of different individuals gets very different ; for 

 just as each species of the animal kingdom has its own specific 

 E, so likewise has each occupation. Contrast the E of the 

 sedentary brain-toiler and of the unskilled muscular worker. 

 The marked difference in the E of each is sufficiently attested 

 by the different ways in which the S has become moulded in 

 each case, for I may anticipate a part of the next chapter by 

 observing that E is capable of working a marked effect upon S, 

 and what a difference there will be in the S of these two indi- 

 viduals ! In the one the muscles will be flabby, the skeleton 

 light, the heart small, the blood-vessels thin ; in the other 

 the muscles will be firm, hard, and hypertrophied, the bones 

 thick, the surfaces for muscular attachment rough and promi- 

 nent, the heart hard, and the valves and blood-vessels thickened; 

 in the one the higher nervous centres will be developed to 

 the furthermost limits, while in the other they will remain very 

 largely in statu quo. These are but a few of the structural differ- 

 ences in each case, and these differences are wrought through E. 

 The influence of education must be said to belong to E, for, as 

 we have seen, it essentially comes through E, and the same 

 may be said of muscular exercise. Muscular exercise is a 

 working against gravity, and this is essentially an external 

 force. When the weight lifted is not part of the body itself, 

 this is quite evident ; when it consists of the body, or part of 

 it, some of the force to be overcome lies within the body, since 

 the latter and the earth mutually act upon one another, but 

 the share taken by the body in producing the weight is of 

 course infinitesimal. Indeed, the effect of use and disuse on 

 body structure is in the last resort always an effect wrought 

 through the external E ; and I desire to lay particular stress 

 on this fact, for I shall utilize it in support of a hypothesis 

 to be subsequently advanced regarding the cause of natural 

 variations. The structural difference between the sedentary 

 scholar and the active labourer is the test and criterion of the 

 different E in each case ; and thus we have palpable evidence 



