90 ORIGIN OF SPECIES 



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 consti- 

 tution, 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 cli- 

 mates ! In Russia the small Asiatic cockroach has every- 

 where 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 

 from the foregoing remarks, namely, that the structure of 

 every organic being is related, in the mose essential yet often 

 hidden manner, to that of all the other organic beings, with 

 M'^hich 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 parasite which clings 

 to the hair on the tiger's body. But in the beautifully plumed 

 seed of the dandelion, and in the flattened and fringed legs 

 of the water-beetle, the relation seems at first confined to 

 the elements of air and water. Yet the advantage of plumed 

 seeds no doubt stands in the closest relation to the land being 

 already thickly clothed with other plants ; so that the seeds 

 may be widely distributed and fall on unoccupied ground. 

 In the water-beetle, the structure of its legs, so well adapted 



