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, 
afew years ago, an insect pest called the cottony cushion 
scale (Icerya purchast). 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 
