STRUGGLE FOR EXISTENCE 59 



of the action and reaction 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 para- 

 site 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 

 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 sup- 

 plant 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 de- 

 crease 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 dif- 

 ferent 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 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 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 constitution, 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. We see this in the recent 

 extension over parts of the United States of one species of swal- 

 low, having caused the decrease of another species. The recent 



