Chap. III.] STRUGGLE FOR EXISTENCE. 59 



food, and are exposed to the same dangers. In the case of varieties 

 of the same species, the struggle will generally he almost equally 

 severe, and we sometimes see the contest soon decided : for instance, 

 if several varieties of wheat he sown together, and the mixed seed he 

 resown, some of the varieties which hest suit the soil or climate, or are 

 naturally the most fertile, will heat the others and so yield more 

 seed, and will consequently in a few years supplant the other varie- 

 ties. To keep up a mixed stock of even such extremely close varie- 

 ties as the variously-coloured sweet-peas, they must he each year 

 harvested separately, and the seed then mixed in due proportion, 

 otherwise the weaker kinds will steadily decrease in number and 

 disappear. So again with the varieties of sheep : it has heen 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 out 

 domestic plants or animals have so exactly the same strength, 

 habits, and constitution, that the original proportions cf 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 Darts cf 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. One species of char- 

 lock 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 

 ono species has been victorious over another in the great battle of life. 



