92 . STRUGGLE FOR EXISTENCE. [Chap. HL 



other, or on the trees, their seeds and seedKngs, or on 

 the other plants which first clothed the ground and 

 thus cheeked the growth of the trees! Throw up a 

 handful of feathers, and all fall to the ground according 

 to definite laws; but how simple is the problem where 

 each shall fall compared to that of the action and re- 

 action 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 ex- 

 posed to the same dangers. In the case of varieties 

 of the same species, the struggle will generally be al- 

 most 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 with the varieties 



