116 THE OHIO IN OF SPECIES 



original proportious of u iiiixed stock (crossing being 

 prevented) could be kept up for half a. dozen genera- 

 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. 



Struggle for Life most severe between Individuals and 

 Varieties of the same Species 



I 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 

 j 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 hav- 

 ing 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 difierent climates I In Russia 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. 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. 



A corollary of the highest importance may be deduced 



