Darwin's On the Origin of Species. Daniel Duzdevich

one will starve out the others. The same result follows when keeping different varieties of medicinal leeches together. It is unlikely that varieties of any domestic plants or animals have so exactly the same strengths, habits, and constitutions that the original proportions of a mixed stock could be kept up for half a dozen generations if they were allowed to struggle as in the wild and without annual sorting of the seed or young.

      Because species within the same genus are always similar in structure and usually in habit and constitution, the struggle between its members will generally be more severe than between members of different genera when they come into competition with one another. The recent extension of one swallow species in the United States caused the decrease of another swallow species; the recent increase of mistle thrush in parts of Scotland caused the decrease of the song thrush; across Russia, the small Asiatic cockroach has driven out a larger species; one species of charlock will supplant another; and we often hear that in many different climates one rat species takes the place of another. We can dimly see why competition should be most intense between closely related forms that fill similar niches. But there is probably no case in which we can know precisely why one species has been victorious over another in the great battle of life.

      An important corollary can be deduced from these remarks: the structure of every organism is related in an essential but often hidden way to that of every other organism with which it competes for food or space, it preys on, or it must escape from. This is obvious in the structures of the teeth and claws of the tiger, and in the legs and claws of the parasite clinging to its hair. In the beautifully plumed seeds of the dandelion and in the flattened and fringed legs of the water beetle, the relationship seems at first confined to the elements of air and water. But the advantage of plumed seeds is no doubt due to the land already being thickly crowded by plants, so such seeds can be broadly distributed and fall on unoccupied ground. A water beetle can compete with other aquatic insects, hunt prey, and elude predators because the structure of its legs is so well adapted for diving. The store of sustenance in seeds may appear to have no relationship to other plants, but judging by the strong growth of young plants produced from such seeds (like peas and beans), I suspect that large stores of sustenance favor the growth of young seedlings struggling with vigorous plants already growing all around.

      Why doesn’t a plant in the middle of its range double or quadruple in number? It can clearly withstand slightly warmer, colder, dryer, or damper conditions, because in other places it ranges into such areas. If we want the plant to proliferate in this hypothetical case, we must endow it with some advantage over its competitors or the animals that feed on it. A constitutional change that helps it endure climate may suffice at the periphery, but only a few plants or animals range so far that that they are destroyed by climate alone. Only in extreme conditions, like in the Arctic or a total desert, will competition cease. Even in extremely cold or dry lands there is competition between a few species or between individuals of the same species for the warmest or dampest spots.

      Thus if a plant or animal is transplanted to a new region with new competitors, its environment will be fundamentally different even if the climate is identical to that of its former home. The hypothetical modification necessary to increase its average numbers is not the same in the new location as in the old; some advantage over different competitors or enemies is needed.

      It is useful to try to imagine how one form could be endowed with an advantage over another, even though there is probably no case where we would know what to do so as to succeed. This exercise will convince us of our ignorance of the mutual relationships among organisms, a conviction as necessary as it is difficult to acquire. Keep in mind that each organism strives to increase geometrically; that each at some period of its life, during some season of the year, or during each generation or at intervals, struggles for existence, and suffers great destruction. When we reflect on this struggle, we may console ourselves with the belief that the war of nature is not incessant, that no fear is felt, that death is generally prompt, and that the vigorous, healthy, and happy survive and multiply.

      1. [The “scale of nature” is an archaic system of ordering all beings, single-file, from the “highest” to the “lowest.” Darwin and his naturalist contemporaries seem to have used it in a less strict sense to characterize organisms based on complexity. – D.D.]