66 STRUGGLE FOR EXISTENCE. 



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 sometimes the case 

 with those which may be strictly said to struggle with each 

 other for existence, as in the case of locusts and grass-feed- 

 ing quadrupeds. But the struggle will almost invariably be 

 most severe between the individuals 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 varie- 

 ties 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-pease, 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 varie- 

 ties 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 original propor- 

 tions 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 manner as beings in a state 

 of nature, and if the seed or young were not annually pre* 

 served 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 constitu* 

 tion, 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. 



