70 STRUGGLE FOR EXISTENCE. 



species, for they frequent the same districts, require the 

 same food, and are exposed to the same dangers. In the 

 case of varieties of the same species, the struggle will 

 generally be almost equally severe, and we sometimes see 

 the contest soon decided: for instance, if several varieties 

 of wheat be sown together and the mixed seed be resown, 

 some of the varieties which best suit the soil or climate, 

 or are naturally the most fertile, will beat the others 

 and so yield more seed, and will consequently in a 

 few years supplant the other varieties. To keep up a 

 mixed stock of even such extremely close varieties as 

 the variously colored sweet peas, they must be 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 been 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 difEerent 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 man- 

 ner 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 



