92 STEUGGLE FOE EXISTENCE. [Chap. III. 



on each other, or on the trees, their seeds and seedlings, 

 or on the other plants which first clothed the gronnd 

 and thus checked the growth of the trees ! Throw up a 

 handful of feathers, and all fall to the ground according 

 to definite laws ; hut how simple is the 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 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 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 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-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 



