2 88 THE CAUSATION OF DISEASE. 



an E to which, in the course of ages, it has become adapted. 

 This harmonious working between each individual and its 

 specific E is the outcome of personal adaptation and natural 

 selection — of direct and indirect equilibration. What may, 

 therefore, be termed the natural E of an animal is that E to 

 which, in the course of ages, it has become adapted. Now the 

 human race is adaptable to certain forms of artificial E, and 

 w T hen, by natural selection or otherwise, perfect adaptation to 

 any artificial E is effected, that E becomes in its turn quite as 

 " natural " as the E of the savage. A savage, if he understood 

 his relation to his comparatively near relatives, the apes, would 

 no doubt think his E very artificial as compared with theirs. 



To sum up. the only criterion of " naturalness " is the degree 

 of adaptation ; perfect naturalness means perfect adaptation, 

 and thus it may well be that an E w T hich we regard to-day as 

 highly artificial — such, for instance, as the artificial feeding of 

 children — will in the future come to be looked upon as quite 

 natural. 



Although I have frequently used the term " artificial " E, I 

 have not yet attempted a definition of it. The best definition 

 I can give is : u An E determined by an elaborate reason." If 

 we did not qualify the word " reason " in some way, it would 

 follow from the definition as it would then stand that the E 

 of the most primitive savage, and of many of the higher 

 animals (I exclude the domesticated animals) w T ould come 

 under the head of artificial E's. The definition is, however, 

 very imperfect, from the fact that " elaborate " is, in common 

 with most epithets, a comparative term, but, in truth the case 

 admits of no better, inasmuch as " artificial " is also compara- 

 tive, and the thing to which it is attached, therefore, does not 

 admit of an absolute definition. 



As w T e ascend the scale of life, we find the enviro-regulating 

 apparatus becoming more and more complex. Starting as a 

 simple, undifferentiated nervous system, it passes through the 

 successive phases of reflex mechanism, conscious reflex 

 mechanism (or simple instinct), and finally, reasoning reflex 

 mechanism, the latter obtaining only in man and the higher 

 brutes. And, side by side w T ith this growing complexity of 

 the E -regulating system, there is a corresponding increase in 



