THE THEORY OF NATURAL SELECTION. 287 



Nor can it be reasonably doubted that in the case of 

 animals this is the process by which varieties are 

 originated and established. But it has been previously 

 seen that * species ' and ' varieties ' pass into one another 

 by imperceptible gradations. It is, in fact, impossible 

 to lay down any fixed rule for the determination of 

 where a ' variety ' ends, and where a ' species ' begins. 

 If, therefore, it be admitted that ' varieties ' are produced 

 by 'natural selection,' it is not possible to deny that 

 the same cause must have given rise to at any rate 

 some of those groups of individuals which naturalists call 

 ' species.' If this be conceded, it is an inevitable logical 

 conclusion that all species have been thus produced by 

 ' natural selection.' At any rate, the admission that any 

 species have been produced by the operation of ' natural 

 selection,' throws upon those who deny the universal 

 operation of the law, the burden of proof that any 

 particular species has not been produced by the action 

 of the same law. 



The above may be taken as a brief statement of the 

 principal propositions upon which Darwin based his 

 celebrated theory of the Origin of Species by means of 

 Natural Selection. This statement would, however, be 

 incomplete without a short additional exposition of what 

 Mr Darwin has called ' artificial selection.' In the case, 

 then, of our domestic animals and their innumerable 

 varieties, there is the obvious fact that the law of ' natural ' 

 selection is prevented from operating in its entirety 

 owing to the action of man. Man, in the case of his 

 domestic animals, steps in as a deus ex machina, and 

 more or less efficiently interferes with the law of natural 



