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Show 76 STRUGGLE FOR EXISTENCE. CHAP. Ill. tion, otherwise the weaker kinds will steadily decrease in numbers and disappear. So again with the varieties of sheep: it has been asserted that certain mountainvarieties will starve out other mountain-varieties, so that they cannot be kept together. The same result has followed from ke.eping 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 char lock will supplant another, and so in other cases. vVe can dimly see why the con1- petition 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. CHAP. III. STRUGGLE FOR EXISTENCE. 77 ·A corollary of the highest importance may be deduced from the foregoing remarks, namely, that the structure of every organic being is related, in the most essential ye~ often. hidden manner, to that of all other organic bmngs, With which it comes into competition for food or residence, or from which it has to escape, or on which it preys. This is obvious in the structure· of the teeth and talons of the tiger ; and in that of the legs and claws of the parasite which clings to the hair on the· tiger's body. But in the beautifully plumed seed of the dandelion and in the flattened and fringed legs of the water-beetle: the relation seems at first confined to the elements of air and water. Yet the advantage of plumed seeds no doubt stands in the closest relation to the land being already thickly clothed by other plants; so that the seeds may be widely distributed and fall on unoccupied ground. In the water-beetle, the structu.re of its legs, so well adapted for diving, allows it to compete with other aquatic insects, to hunt for its own prey, and to escape serving as prey to other animals. The store of nutriment laid up within the seeds of many plants seems at first sight to have no sort of relation to other plants. But from the strong growth of young plants produced from. such seeds (as peas and beans), when sown in the midst of long grass, I suspect that the chief use of the nutriment in the seed is to favour the growth of the young seedling, whilst struggling with other plants growing vigorously all around. Look at a plant in the midst of its range, why does it not double or quadruple its numbers? We know that it can perfectly well withstand a little more heat or cold, dampness or dryness, for elsewhere it ranges |
