76 STRUGGLE FOR EXISTENCE. Chap. III. 



tion, otherwise the weaker kinds will steadily decrease 

 in numbers and disappear. So again with the varieties 

 of sheep : it has been asserted that certain mountain- 

 varieties will starve out other mountain-varieties, so 

 that they cannot be kept together. The same result 

 has followed from keeping together different varieties 

 of the medicinal leech. It may even be doubted whe- 

 ther the varieties of any one of our domestic plants 

 or animals have so exactly the same strength, habits, 

 and constitution, that the original proportions of a 

 mixed stock could be kept up for half a dozen genera- 

 tions, if they were allowed to struggle together, like 

 beings in a state of nature, and if the seed or young 

 were hot annually sorted. 



As species of the same genus have usually, though 

 by no means invariably, some similarity in habits and 

 constitution, and always in structure, the struggle will 

 generally be more severe between species of the same 

 genus, when they come into competition with each other, 

 than between 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 an- 

 other 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 climates! In Eussia the small Asiatic 

 cockroach has everywhere driven before it its great 

 congener. One species of charlock will supplant another, 

 and so in other cases. We can dimly see why the com- 

 petition 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. 



