62 



EVOLUTION AND ANIMAL LIFE 



crowded aside or trampled on by its associates. It is the least 

 adaptable which suffers most from extremes of heat and cold. 

 By the process of Artificial Selection the breeder improves his 

 stock, destroying his weakest or least comely calves, reserving 

 the strong and fit for parentage. In like fashion, on an in- 

 conceivably large scale, the forces of nature are at work modify- 

 ing and fitting to the demands of their surroundings the different 

 species of animals. Because the processes and results of the 

 struggle for existence seem parallel with those of artificial 

 selection, Darwin suggested the name of Natural Selection 

 for the sifting process as seen in nature. To the general re- 

 sult of natural selection, Herbert Spencer has applied the term 



FIG. 39. The Australian ladybird, Vedalia cardinalis, feeding on cottony cushion scale, 



Icerya purchasi. (From life.) 



Survival of the Fittest. By fitness in this sense is meant only 

 adaptation to surrounding conditions, for the process of natural 

 selection has no necessary moral element, nor does it necessarily 

 work toward progress among organisms. With changing con- 

 ditions species undergo change. Some individuals, by the 

 possession of slight advantageous variations of structure or of 

 instinct, meet these new demands better than others. These 

 survive, the others die. The survivors produce young sharing 

 in part, at least, their own advantages, and with renewed selec- 

 tion the degree of adaptation increases with successive genera- 

 tions. 



To the process of natural selection we must, in most cases, 

 probably ascribe the adjustment of species to surroundings. 



