Chap. III. MUTUAL CHECKS TO INCKEASE. 75 



must here have gone on during long centuries, each an- 

 nually scattering its seeds by the thousand ; what war 

 between insect and insect — between insects, snails, and 

 other animals with birds and beasts of prey — all striving 

 to increase, and all feeding on each other or on the trees 

 or their seeds and seedlings, or on the other plants which 

 first clothed the ground and thus checked the growth of 

 the trees ! Throw up a handful of feathers, and all must 

 fall to the ground according to definite laws ; but how 

 simple is this problem compared to the action and re- 

 action of the innumerable plants and animals which have 

 determined, in the course of centuries, 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 often the case 

 with those which may strictly be said to struggle with 

 each other for existence, as in the case of locusts and 

 grass-feeding quadrupeds. But the struggle almost in- 

 variably will 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 quite sup- 

 plant 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 tl e seed then mixed in due propor- 



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