CiiAr. 111. STRUGGLE FOlt EXISTENCE. 81 



exposed to the same dangers. lu the case of vai-ietics 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 Avhich best 

 suit the soil or climate, or are naturally the most fertile, Avill 

 beat the others and so yield more seed, and will consequently 

 in a few years quite su^iplant the other varieties. To keep vip 

 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, other- 

 wise the weaker kinds Avill steadily decrease in number and 

 disappear. So, again, with the varieties of sheep : it has licen 

 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 varie- 

 ties 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 could be kept up for half 

 a dozen generations, if they were allowed to struggle together, 

 like beings in a stUte of nature, and if the seed or young were 

 not annually sorted. 



' Struggle for Life most severe between Individuals and Vari- 

 eties of the same Sjyecles. 



As species of the same genus have usually, though by no 

 means invariably, much 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 Avith each other, than between species of dis- 

 tinct 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 differ- 

 ent climates ! In Russia the small Asiatic cockroach has every- 

 where driven before it its great congener. In Australia the 

 imported hivG^bce is rapidly exterminating the small, .stingless 

 native bee. One species of charlock has been known to sup- 

 plant another species ; and so in other cases. We can dimly 



