CHAPTER XXV 



PKACTICAL PROBLEMS 



Physical deterioration The influence of urban conditions on the race 

 Public health Water-, Earth-, and Air-borne diseases In- 

 temperance Child-birth Insanity. 



518. THE principal conclusion we have reached in our 

 study of heredity is the one that variations are very rarely 

 caused by the direct action of the environment on the germ- 

 plasm so rarely that racial change is never due to this 

 cause. Were the fact otherwise, racial adaptation to the 

 environment, and therefore, the persistence of life, would be 

 impossible. Practically speaking, therefore, all variations, 

 except those due to the regressive action of bi-parental 

 reproduction, are spontaneous. It follows, as a necessary 

 consequence, that all progressive racial change is due to 

 selection, whereas apart from the effects of reversed selection, 

 all regressive racial change is an accompaniment of cessation 

 of selection. We saw how perfectly the adaptation of the 

 various races of mankind to a multitude of diseases and 

 to the hard conditions of civilized life, accords with this 

 hypothesis. 



519. Of late years the public mind has been exercised on 

 the subject of physical deterioration. We are told that, since 

 a large and increasing proportion of the population have 

 become town-dwellers living under bad hygienic conditions, 

 the race is undergoing degeneration. The terms " deteriora- 

 tion " and " degeneration," however, are used in a very vague 

 way. If it be meant that bad and insufficient food, over- 

 crowding, overwork, lack of light and air, prevalence of 

 disease and intemperance, and the like, render the inhabitants 

 of slums individually less robust than their country cousins, 

 then the statement is obviously true. Of course an in- 

 dividual reared under conditions unfavourable to his 

 development tends to be less robust than one reared under 



