Survival of the Fittest. 435 



account for the action and reaction of the innumerable plants 

 and animals that have determined in the course of untold 

 centuries the proportional numbers and kinds of trees that 

 are now found growing on these old Indian ruins. But the 

 struggle will almost invariably be the severest between 

 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. If several 

 varieties of wheat be sown together, and the mixed seed be 

 re-sown, 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 others. Such extremely-close varieties as the 

 variously-colored sweet-peas must be separately harvested 

 each year, and the seed mixed in due proportion, or the 

 weaker kinds will steadily decrease in number and disap- 

 pear. So, again, with the varieties of sheep. Certain 

 mountain-varieties will starve out other mountain-varieties, 

 so that they cannot be kept together. Similar results have 

 followed from keeping together different varieties of the 

 medicinal leech. In view of all that has been said, it is 

 questionable whether the varieties of any of our domestic 

 plants and animals have so exactly the same vigor, constitu- 

 tion and habits that the original proportions of a mixed 

 stock could be kept up for a half-dozen generations if they 

 were permitted to struggle together like beings in a state of 

 nature, if the seed or young were not annually assorted. 



Species of the same genus having usually, though not 

 invariably, much similarity in habits and constitution, and 

 always in structure, the struggle will be more severe between 

 species of the same genus, where they come into competition 

 with each other, than between species of distinct genera. 

 One species of swallow has caused in certain parts of the 

 United States the decrease of another species, just as the 

 missel-thrush in parts of Scotland has caused the decrease of 



