Chap. III.] STRUGGLE FOB EXISTENCE. 93 



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 whether 

 the varieties of any of our domestic plants or animals 

 have so exactly the same strength, habits, and constitu- 

 tion, that the original proportions of a mixed stock 

 (crossing being prevented) could be kept up for half-a- 

 dozen generations, if they were allowed to struggle to- 

 gether, in the same manner as beings in a state of nature, 

 and if the seed or young were not annually preserved in 

 due proportion. 



Struggle for Life most severe'ietween Individuals and 

 Varieties of the same Species. 



As the species of the same genus usually have, 

 though by no n^ans invariably, much similarity in 

 habits and constitution, and always in structure, the 

 struggle will generally be more severe between them, if 

 they come into competition with each other, than be- 

 tween 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 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 cock- 

 roach has everywhere driven before it its great con- 

 gener. In Australia the imported hive-bee is rapidly 

 exterminating the small, stingless native bee. One spe- 



