STRUGGLE FOR EXISTENCE 115 



ries, the proportional numbers and kinds of trees now 

 growing on the old Indian ruins! 



The dependency of one organic being on another, as 

 of a parasite on its prey, lies generally between beings 

 remote in the scale of nature. This is likewise some- 

 times the case with those which may be strictly said to 

 struggle with each other for existence, as in the case 

 of locusts and grass-feeding quadrupeds. But the struggle 

 will almost invariably be most severe between the indi- 

 viduals of the same 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 num- 

 ber 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 of our domestic plants or animals have so exactly 

 the same strength, habits, and constitution, that the 



