Darwin s Artificial and Natural Selection 115 



stitution, and always in structure, the struggle will generally 

 be more severe between them, if they come into competition 

 with each other, than between the species of distinct genera. 

 We see this in the recent extension over parts of the United 

 States of one species of swallow having caused the decrease 

 of another species. The recent increase of the missel-thrush 

 in parts of Scotland has caused the decrease of the song- 

 thrush. How frequently we hear of one species of rat taking 

 the place of another species under the most different cli- 

 mates ! In Russia the small Asiatic cockroach has every- 

 where driven before it its great congener. In Australia the 

 imported hive-bee is rapidly exterminating the small, sting- 

 less native bee. One species of charlock has been known to 

 supplant another species ; and so in other cases. We can 

 dimly see why the competition should be most severe 

 between allied forms, which fill nearly the same place in the 

 economy of nature ; but probably in no one case could we 

 precisely say why one species has been victorious over 

 another in the great battle of life." 



All this goes to show, if it really shows anything at all, 

 that the distribution of a species is determined, in part, by 

 its relation to other animals and plants — a truism that is 

 recognized by every naturalist. The statement has no neces- 

 sary bearing on the origin of new species through competi- 

 tion, as the incautious reader might infer. Not that I mean 

 in any way to imply that Darwin intended to produce this 

 effect on the reader ; but Darwin is not always careful to 

 discriminate as to the full bearing of the interesting illustra- 

 tions with which his book so richly abounds. 



At the end of his treatment of the subject, Darwin empha- 

 sizes once more how little we know about the subject of the 

 struggle for existence. 



" It is good thus to try in imagination to give to any one 

 species an advantage over another. Probably in no single 

 instance should we know what to do. This ought to con- 



