Ufl. X.] NATURAL SELECTION. 17 



of the blood. And the stomach, intestines, and various 

 eecreting glands will respond to the requirements of all these 

 nutritive changes. While, lastly, such deep-seated variations 

 cannot fail to influence the nervous system of the animal, and 

 to modify somewhat its temperament and its modes of life. 



To illustrate the effects of use and disuse, let us reconsider 

 the antelopes, of whom natural selection has so long pre« 

 served the swiftest and most quickly frightened individuals 

 that they now rank among the fleetest and most timid of 

 mammals. If all the lions and other swift caruivora of 

 Africa were to become extinct, so that antelopes would no 

 longer have to run for their lives, the slower and less easily 

 alarmed individuals would begin to be preserved in as great 

 numbers as the swifter and more timid ones, so that by and 

 by the average speed and timidity of the race would be 

 diminished. In all this we see merely the effects wrought by 

 unaided natural selection. But it is a fundamental law of 

 biology that functions are maintained at their maximum only 

 through constant exercise. Freed from savage enemies, our 

 antelopes would less frequently use the muscles concerned in 

 running, and would less often exercise the mental faculties 

 concerned in the rapid perception of approaching danger. 

 Inevitably, therefore, they would, after several generations, 

 diminish in speed, and become less alert and less timid. 

 Here we see the effects of what is called the law of use and 

 disuse. But to these we should also have to add the effects 

 of correlation of growth. Decrease in speed, involving 

 decrease in muscular tonicity, and rendering possible the 

 assimilation of less concentrated food, would seriously modify 

 the nutrition of the entire organism. The digestive tract 

 would probably be enlarged, and larger and lazier bodies 

 could not fail to be produced, both by the direct influence of 

 the nutritive processes, and because natural selection would 

 uo longer necessitate the slaughter of all clumsy-bodied 

 individuals. Thus in course of time the breed of antelopes 



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