THE CAUSATION OF DISEASE. 1 87 



the unfit and a survival of the fit, and thus by a gradual 

 accumulation, during long periods of time, of favourable 

 variations, species are in large measure evolved, the ultimate 

 result being that each creature tends to become more and 

 more improved in relation to its external conditions, and 

 " this improvement eventually leads to the gradual advance- 

 ment of the organization of the greater number of living 

 beings throughout the world, the degree of advance being 

 measured by the completeness of the division of physiological 

 labour." 



The law of " Natural Selection," or " Survival of the 

 Fittest," is one of the greatest natural laws yet discovered, and 

 although its exposition originally met with some opposition, it 

 is now accepted as an uncontrovertible fact. Herbert Spencer 

 says of it : " When once enunciated the truth of the hypothesis 

 is so obvious as scarcely to need proof." It has passed from 

 the realm of hypothesis into that of law. 



I would particularly emphasize the distinction between the 

 struggle for food, or, what is the same thing, the struggle 

 against starvation, and the struggle against the remaining 

 adverse conditions. When we come to speak of man, we shall 

 see what a host of diverse conditions are included under the 

 latter, and that with him — at all events with civilized man — 

 it is as regards these latter that natural selection plays its 

 chief part. 



Sexual Selection consists in the struggle between the mem- 

 bers of one sex for possession of individuals of the other; 

 and it is generally between the males for the female. Success 

 may depend upon general strength and vigour, in which case 

 the strongest and most vigorous males will tend to have the 

 most offspring (we may even assume that such a struggle 

 takes place between myriads of spermatozoa for the possession 

 of the ovum, the hardiest and lustiest proving successful) ; or it 

 may depend upon the possession of special weapons, such as 

 the horns of the stag, or the spurs of the cock, which, there- 

 fore, tend to increase from generation to generation; or, finally, 

 success may depend upon freedom of choice. The beautiful 

 plumage of cock birds is due to a selection by the hens of 



