STRUGGLE FOR EXISTENCE 89 



to the ground according to definite laws; but how simple is 

 tlie problem where each shall fall compared to that of the 

 action and reaction of the innumerable plants and animals 

 which have determined, in the course of centuries, the pro- 

 portional 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- 

 feeding 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 varie- 

 ties of the same species, the struggle will generally be almost 

 equally severe, and we sometimes see the contest soon de- 

 cided : 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-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 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 different varieties of the 

 medicinal leech. It may even be doubted whether the varie- 

 ties of any of our domestic plants or animals have so ex- 

 actly the same strength, habits, and constitution, that the 

 original proportions of a mixed stock (crossing being pre- 

 vented) 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 preserved in due proportion. 



