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 invariably 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 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 numbers 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 different varieties of the medicinal leech.  It may even be doubted whether the varieties of any one of our domestic plants or animals have so exactly the same strength, habits, and constitution, that the original proportions of a mixed stock could be kept up for half a dozen generations, if they were allowed to struggle together, like beings in a state of nature, and if the seed or young were not annually sorted.

As species of the same genus have usually, though by no means invariably, some similarity in habits and constitution, and always in structure, the struggle will generally be more severe between species of the same genus, when they come into competition with each other, than between species of distinct genera.  We see this in the recent extension over parts of the United States of one species of swallow having caused the decrease of another species.  The recent increase of the missel-thrush in parts of Scotland has caused the decrease of the song-thrush.  How frequently we hear of one species of rat taking the place of another species under the most different climates!  In Russia the small Asiatic cockroach has everywhere driven before it its great congener.  One species of charlock will supplant another, and so in other cases.  We can dimly see why the competition should be most severe between allied forms, which fill nearly the same place in the economy of nature; but probably in no one case could we precisely say why one species has been victorious over another in the great battle of life.