206 THE CAUSATION OF DISEASE. 



the checks to increase being more or less inoperative among 

 them ; indeed, since the advance of sanitary science, the popula- 

 tion in some cases has been increasing by leaps and bounds, 

 and in such countries natural selection cannot be so rigorous 

 as among those savage communities where the population is 

 stationary. 



The checks to increase consist of such violent agencies as 

 famine, warfare, accident, and infanticide, or of the more in- 

 sidious operations of bad hygienic surroundings, and the many 

 different forms of disease which result therefrom. 



Among savage tribes the larger proportion of deaths is 

 probably due to violent causes rather than to disease, but the 

 latter is certainly the chief cause of death among civilized 

 peoples, although warfare still plays a sadly prominent part. 



Let us now see how far these checks to increase result in the 

 survival of the fittest. 



All deaths occurring before or during the procreative period 

 of life, save such as are due to purely accidental causes, fall under 

 the law of natural selection, for they are deaths of the unfit. 

 Unfit individuals are thus weeded out and prevented entirely 

 or in part from transmitting their unfitness to posterity, while 

 the fitter survive and propagate the species. 



By u purely accidental causes," I mean such as would in- 

 evitably lead to the death of anybody exposed to them, and 

 which are, moreover, in no way due to personal carelessness. 

 For, if the mental self be responsible for the death, the individual 

 may be regarded as mentally unfit, the fault residing largely 

 in himself ; and if the E be one which would not cause death 

 in every one, death cannot in that case be said to be strictly 

 accidental, since it is due to an individual weakness as regards 

 a special E,* a weakness which is not shared by all others. 

 If, therefore, personal weakness, mental or physical, plays a 

 part in causing death, we cannot say that the death is due to 

 accident, and in all such cases adaptation is possible by a sur- 

 vival of the fittest. 



The causes which inevitably lead to the death of each and 

 every individual coming under their influence are violent and 

 non-violent : the former include, among others, traumatism, 

 * See chapter on " Normality of S and E," Part III. 



