THE STRUGGLE FOR EXISTENCE 121 



desired by a force as unchanging as that by which a stream 

 turns a mill. 



From the wild animals about him man has developed 

 the domestic animals which he finds useful. The dog 

 which man trains to care for his sheep is developed by 

 selection from the most tractable progeny of the wolf which 

 once devoured his flocks. By the process of artificial selec- 

 tion those individuals that are not useful to man or pleas- 

 ing to his fancy have been destroyed, and those which con- 

 tribute to his pleasure or welfare have been preserved and 

 allowed to reproduce their kind. The various fancy breeds 

 of pigeons the carriers, pouters, tumblers, ruff-necks, and 

 fan-tails are all the descendants of the wild dove of Eu- 

 rope (Columba livia). These breeds or races or varieties 

 have been produced by artificial selection. So it is with 

 the various breeds of cattle and of hogs and of horses 

 and dogs. 



In this artificial selection new variations are more rap- 

 idly produced than in Nature by means of intercrossing 

 different races, and by a more rapid weeding out of un- 

 favorable that is, of undesirable variations. The rapid 

 production of variations and the careful preservation of 

 the desirable ones and rigid destruction of undesirable 

 ones are the means by which many races of domestic ani- 

 mals are produced. This is artificial selection. 



73. Dependence of species on species. There was intro- 

 duced into California from Australia, on young orange trees, 

 a few years ago, an insect pest called the cottony cushion 

 scale (Iccrya purchasi). This pest increased in numbers 

 with extraordinary rapidity, and in four or five years threat- 

 ened to destroy completely the great orange orchards of 

 California. Artificial remedies were of little avail. Finally, 

 an entomologist was sent to Australia' to find out if this 

 scale insect had not some special natural enemy in its 

 native country. It was found that in Australia a certain 

 species of lady-bird beetle attacked and fed on the cottony 



