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 
