CONCLUSION. 251 



and this is perhaps included in the second and main 

 cause ; namely, the decrease in the internal changes 

 of the body, produced by the continued sameness of 

 the stimuli acting upon it. If the continued same- 

 ness of environment tends to weaken the reactions 

 and processes of life and bring on senescence and 

 death, then we would expect that changes of envi- 

 ronment would strengthen the reactions and exert a 

 favourable influence upon the processes of life. The 

 following passages from Darwin's Origin of Species 

 gives the reply to this supposition : " It is an old 

 and almost universal belief, founded on a consider- 

 able body of evidence, which I have elsewhere given, 

 that slight changes in the conditions of life are bene- 

 ficial to all living beings. We see this acted on by 

 farmers and gardeners in their frequent exchanges of 

 seed, tubers, etc., from one soil or climate to another, 

 and back again. During the convalescence of ani- 

 mals, great benefit is derived from almost any change 

 in their habits of life . . , the principle of life, ac- 

 cording to Mr. Herbert Spencer, being that all life 

 depends on, or consists in, the incessant action and 

 reaction of various forces, which, as throughout 

 nature, are always tending towards an equilibrium; 

 and when this tendency is slightly disturbed by any 

 change, the vital forces gain in power." 



In concluding this brief exposition of the theory 



