70 ON THE ORIGIN OF 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 consequently in a few years quite supplant 

 the other varieties. To keep up a mixed stock of even 

 such extremely close varieties as the variously coloured 

 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 numbers 

 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 different varieties of the 

 medicinal leech. It may even be doubted whether the 

 varieties of any one of our domestic plants or animals 

 have so exactly the same strength, habits, and con- 

 stitution, that the original proportions of a mixed 

 stock could be kept up for half-a-dozen generations, if 

 they were allowed to struggle together, like beings in a 

 state of nature, and if the seed or young were not 

 annually sorted. 



As species of the same genus have usually, though 

 by no means invariably, some similarity in habits and 

 constitution, and always in structure, the struggle will 

 generally be more severe between species of the 

 same genus, when they come into competition with 

 each other, than between 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 climates ! In Russia the 

 ■mall Asiatic cockroach has everywhere driven before it 

 its great congener. One species of charlock will tup- 



