individuals of a species are just alike. Each varies in some slight respect 

 from the ty]>e. Of these variations, some may be indilferent, some use- 

 ful, some harmful. According to these authors, these variations may 

 affect all parts of the body, the form, the size and strength of single 

 organs, color, or mental ({ualities. Again, all species tend to increase 

 beyond the limits of space and food supply. From this latter cause there 

 arises between the members of any species a struggle for existence. More- 

 over, all species are warred ui)on by many others, by which their food is 

 appropriated and through which they themselves may be appropriated 

 as food. In such a dire struggle it is,. on the average, the best endowed 

 individuals that will succeed in maintaining themselves and in produc- 

 ing offspring to inherit their useful characters ; that is, the most vigorous 

 individuals, those which have developed in the highest degree weapons 

 of offense and defense, or protective colors, or the greatest cunning. The 

 weakest, the most exposed, the most stupid, will perish and leave few or 

 no young. From all the young produced by every species there is thus a 

 constant and unsparing selection being made in favor of those individ- 

 uals which can best endure the stress of the conditions. Hence the 

 meaning of Darwin's phrase " natural selection,'' and of that used by 

 Spencer, " survival of the fittest." Through the selection, for many gen- 

 erations, of the individuals possessing certain beneficial characters, these 

 at length become fixed in the organization and strengthened until the 

 organism is no longer what it was, but may have departed widely there- 

 from. Since success in the struggle is constantly demanding greater 

 strength of limb and body, more efficient organs for each function, more 

 weapons for assailing and repelling, more perfectly protective coloration, 

 the general tendency of evolation has been upward ; but the vigor with 

 which the battle is waged may result in driving some species into such 

 situations that degeneration may occur. Such are many burrowing ani- 

 mals and most parasites. 



This process of natural selection is therefore quite similar to the artifi- 

 cial selection which is practiced by breeders in their effort to develop new 

 varieties of animals and plants. Those individuals are selected which 

 possess in the highest degree the desired quality ; they are crossed with 

 others having, if possible, the same quality, and the offspring of the pair 

 are treated in the same manner, until the character sought is fully devel- 

 oped. 



The rigorousness of the selective process that is going on in nature can 



