2 82 THE CAUSATION OF DISEASE. 



religious — an origin clue in large measure to the subtle play 

 of the human mind — and what will determine the decay of our 

 English nation — if decay there must be — will assuredly not 

 be any change which the physiologist can point to, but one 

 belonging to the region of social and political economics. 



To sum up : — We have seen in the last chapters that direct 

 equilibration (the operation of E upon S independently of 

 selection), indirect equilibration (natural selection or survival 

 of the fittest), and sexual selection are all operative among 

 civilized men, though in varying degree. 



No doubt Herbert Spencer is correct when he tells us that 

 natural selection does not play such a potent part in modifying 

 structure among the higher orders of animals as among the 

 lower. We must, nevertheless, not neglect the part played by 

 it, and from the pathological point of view I venture to think 

 that its operation is still of the utmost importance. In order 

 to realize adequately how important a part natural selection 

 has taken in the evolution of our race, we must bear in mind 

 that each one of us has an ancestiy stretching far back into 

 the dim past, through countless generations of life growing 

 ever simpler till the unicellular protoplasmic unit is reached, 

 and that each individual in this long chain has successfully 

 contended against many competitors — has varied in the right 

 direction. The mind fails to grasp the number that have 

 been unable to struggle against the conditions of their E, and 

 the enormous destruction of life the operation of natural selec- 

 tion has entailed. The present population of the world is as 

 nothing compared with what it would have been had no pro- 

 cess of elimination taken place, always supposing (what is, in 

 fact, impossible) that evolution could have taken place at all 

 without such elimination. 



And as it has been in the past, so it is now. Natural 

 selection is not so active among civilized, as among primitive, 

 communities, a fact which the rapid increase in the population 

 of many civilized countries at once attests. Nevertheless, now 

 as always, individuals are born who are incapable of coping 

 with their E. Often enough — indeed, only too often — the E is 

 a necessarily fatal one — destroying ruthlessly, no matter how 



