STRUGGLE FOR EXISTENCE U81 



tions, 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. 



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 extension over parts of the United 

 States of one species of swallow having caused the 

 decrease of another species. The 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 Rus- 

 sia 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. One species of charlock has 

 been known to supplant another species; and so in 

 other cases. 



A corollary of the highest importance may be de- 

 duced from the foregoing remarks; namely, that 

 the structure of every organic being is related, in 

 the most essential yet often hidden manner, to that 

 of all the other organic beings, with which it comes 

 into competition for food or residence, or from 

 which it has to escape, or on which it preys. This 

 is obvious in the structure of the teeth and talons of 

 the tiger; and in that of the legs and claws of the 



