Chap, in.] STRUGGLE FOR EXISTENCE. 93 



with the varieties of sheep ; it has been asserted that 

 certain mountain-varieties will starve out other moun- 

 tain-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 constitution, 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 together, 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 between Individuals and 

 Varieties of the same Species. 



As the species of the same genus usually have, 

 though by no means 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 

 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 climates ! In Eussia the small 

 Asiatic cockroach has everywhere driven before it its 

 great congener. In Australia the imported hive-bee is 

 rapidly exterminating the small, stingless native bee. 



