THE EVOLUTION OF LIFE 361 



by immense herds of these animals. The descendents of 

 the English sparrow, introduced not many years ago into 

 this country, now number untold millions. 



As only a few of the descendents of any organism can as a 

 rule survive, the chances are that, on the whole, the sur- 

 vivors owe their existence to the possession of some 

 quality which gives them an advantage over their competi- 

 tors in the struggle for existence. As organisms vary, 

 those variations which are best adapted to their conditions 

 of life will, on the whole, survive and propagate their kind. 

 Thus there results in nature a process of selection working 

 ever toward the preservation of the better endowed indi- 

 viduals. This process was called by Darwin natural 

 selection in contradistinction to artificial selection which is 

 practised by man. In natural selection we have a modi- 

 fying agency which is ever tending to mould organisms into 

 better adapted forms. In a herd of wolves for instance it 

 would naturally work to produce greater fleetness of foot, 

 keenness of scent, quickness of eyesight, strength, intelli- 

 gence and other qualities which would give a wolf an ad- 

 vantage over its neighbors. According to Darwin's theory 

 evolution has been brought about mainly through natural 

 selection in a manner more or less similar to that in which 

 man, by a process of artificial selection, in an infinitely 

 shorter period of time, has been able to effect such strik- 

 ing modifications in his cultivated plants and domestic 

 animals. 



