2l6 THE CAUSATION OF DISEASE. 



namely, into physiological and pathological variations. Thus, 

 we have four kinds of variations in all — physiological and 

 pathological mental variations, physiological and pathological 

 bodily variations — and these four may be typified by the 

 following examples — geniuses, idiots, giants, cripples. 



I shall first speak of mental variations, and shall endeavour 

 to discover how far natural selection operates upon them. In 

 my essay on " Instinct and Reason," already quoted, I came to 

 the conclusion that natural selection operated solely on the 

 mental side, and that the selection was not so much a matter 

 of life or death, as the sorting of society into various classes. 

 Both of these conclusions were wrong, for natural selection is 

 not only busily at work on the moral fabric of man — it operates 

 very actively on the physical side also. Moreover, its effect 

 upon the mental side is not only to cause an unequal distribu- 

 tion of wealth, but to lead to an active weeding-out of certain 

 mental types. 



What are these types ? In the first place individuals of 

 distinctly unsound mind have far less chance of leaving off- 

 spring than the sane. They are less likely to be married, and 

 this for many reasons. During the latent stage of insanity, 

 no doubt, some get married, but when once the individual has 

 been actually insane the chances of matrimony are very small. 

 Few, indeed, would marry such an individual. Perhaps there 

 would be less objection among some of the lower classes than 

 among those higher in the social scale, yet, even where the objec- 

 tion may be less strong, there are many checks to such unions. 

 For a large number of the insane are under restraint, and, even 

 if not actually under restraint, they are, for the most part,, 

 placed outside the pale of society. Again, it is a well-known 

 fact that the criminal class is very largely constituted of 

 individuals showing distinct mental deficiency. They come 

 of neurotic families, and are, in fact, pathological variations. 

 Now the checks to the increase of this class are very great, 

 for much of the criminal's life is spent in prison, and a 

 number of the most depraved kind are executed ; indeed, the 

 destruction of these poor wretches in the past has been 

 terrible. 



Further, among the sane, mental aptitude plays a distinct 



