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- 
