1 86 THE CAUSATION OF DISEASE. 



that that there could be no survival of the fittest, since all 

 members of the same species would be the same ; and if 

 there were no struggle for existence, there would be no need 

 of such survival, since all organisms would be adequately 

 provided for. 



Natural variations are, as we have seen, an a priori neces- 

 sity : the E cannot be the same for any two individuals, and, 

 inasmuch as the latter is capable of influencing S, differences 

 of E signify differences of S. Hence there must be a differ- 

 ence even among the individuals reproduced asexually from 

 a single parent. Such differences in E are undoubtedly 

 the primary cause of natural variations j but, once started 

 in this way, they are still further increased by the method of 

 reproduction from two parents, for, in this case, the offspring 

 cannot be exactly like either the one or the other, and, 

 differing from either, will form a natural variation. 



Let us now inquire into the cause of the struggle for exist- 

 ence, and we may limit our observations to the animal 

 kingdom. One great cause is the dearth of food ; for, since 

 organisms tend to increase in geometrical ratio, while the 

 supply of food is limited, a certain number only can secure 

 the proper quantity. There is, consequently, a survival of 

 those best able to do so — of those, namely, which vary in a 

 direction enabling them to compete successfully with their 

 fellows. This struggle is fiercest between individuals of the 

 same species, all of which will be struggling for the same kind 

 of food. 



But there is another element in the struggle; there is a 

 struggle against unfavourable conditions both of the inanimate 

 world, in the shape of excessive heat and cold and so forth, 

 and of the animal world, in the shape of attacks of other 

 animals, parasitic or otherwise. If an individual cannot pro- 

 tect himself against climatic extremes, or the attacks of other 

 animals, he is not properly adapted to his E and succumbs, 

 while such of his fellows as are so happily constructed as to 

 meet the exigencies of their individual E's survive — being 

 fitter or more in harmony therewith. 



It follows that there is a perpetual weeding out of 



