NATURAL SELECTION 15 



so severe any advantage, however slight, of greater vigor, 

 or better structure, may be decisive and turn the scale. 



In these three sets of phenomena, heredity, variation, 

 and the strenuous struggle for existence, we have the 

 basis for progress, for evolution, by the survival of the 

 most perfect individuals. Let us illustrate. 



Among the existing individuals of any species of 

 animal or plant there will be found, at any time, a great 

 variety of more or less divergent forms. Take as an 

 example the common rabbit of eastern America. Some 

 when full grown are larger, some smaller; some are swifter, 

 some run less swiftly ; some are darker colored, some 

 lighter colored ; some are grayish, some more brownish ; 

 some are more shy than the average, some more bold than 

 their fellows ; some are more observant, some less so ; 

 some have greater endurance, some diverge .to the other 

 extreme. So we might go on. Whatever character we 

 choose to observe, we will find it more strongly developed 

 in some individuals than in the rest, and conversely in 

 some it will be developed to less than the average degree. 

 The larger number of individuals in the species will usu- 

 ally pretty closely agree in the extent to which any par- 

 ticular character is developed, but a considerable number 

 will be found who diverge toward either extreme. 



Suppose now there be introduced into the region where 

 these rabbits live some predatory enemy swifter and more 

 sly than those to which the rabbits are now exposed. 

 The first result would be the extermination of those 

 rabbits which are less swift and less cautious and observ- 

 ant. Most of those of average swiftness and alertness 

 also might be caught and killed. There would soon be 



