THE CAUSATION OF DISEASE. 3 I 



shall presently see, for any tivo individuals to be surrounded by 

 exactly the same external-E. That every species — and let us 

 now speak of the animal kingdom only — has a specific external- 

 E is sufficiently obvious. Each species has its own particular 

 haunt, food, and mode of life ; each stands in peculiar rela- 

 tions to numberless other forms of animal life. When, for 

 instance, many animals compete for the same food, each one 

 must include within its E all the others. This struggle is not 

 only between species and species, but between the various 

 members of the same species ; indeed, as Darwin observes, the 

 struggle is always greatest between members of the same 

 species. Many animals, again, prey upon others, and thus 

 both the preying animal and the animals preyed upon are 

 brought into relation with one another, each therefore falling 

 within the E of the other — and among the preying animals we 

 must of course include the parasites. Further, the relation of 

 animal to animal in social communities, as of the ant and bee, 

 is very definite, and the whole remaining colony falls within 

 the organic E of any one ant or bee. Wherefore it is very 

 evident that every animal has a complex living E to 

 which it must adapt itself, or it must die. The wild rabbit, 

 for instance, has to compete against other rabbits, and 

 many other animals in the neighbourhood living upon the 

 same food, and has further to contend against many actual 

 foes, such as man. foxes, stoats, and its own particular body 

 parasites. 



A consideration of the above facts renders it sufficiently 

 obvious that the E of each species must differ considerably ; 

 and, further, that it differs for each individual of the same 

 species, though not to the same extent. No two beings ever 

 have been, nor can they ever be, exactly similarly circumstanced, 

 from the beginning to the end of life. 



In the case of man, the E is still more diverse and complex. 

 By " diverse," I mean that it differs for different individuals ; 

 by " complex," that the forces operating upon the individual 

 are not few and simple, but many and complicated. The im- 

 portance of making this distinction will be evident in another 

 part of this work. In savage communities the E of each 

 individual is comparatively simple, and is very much the 



