PDF - Wallace Online

ix THE DEBT OF SCIENCE TO DARWIN 467plants, hitherto inexplicable, are due to the necessity of keepingaway " unbidden guests," such as snails, slugs, ants, andmany other kinds of animals, which would destroy the flowersor the pollen before the seeds were produced. When thissimple principle is once grasped, it is seen that almost all thepeculiarities in the form, size, and clothing of plants are tobe thus explained, as the spines or hairs of the stem andbranches, or the glutinous secretion which effectually preventsants from ascending the stem, the drooping of theflowers to keep out rain or to prevent certain insects fromentering them, and a thousand other details which are describedin Kerner's most instructive volume. This branch ofthe inquiry was hardly touched upon by Darwin, but it isnone the less a direct outcome of his method and his teaching.The Struggle for ExistenceBut we must pass on from these seductive subjects to givesome indication of the numerous branches of inquiry of whichwe have the results given us in the Origin of Species, butwhich have not yet been published in detail. The observationsand experiments on the relations ofspecies in a state ofnature, on checks to increase and on the struggle for existence,were probably as numerous and exhaustive as those on domesticatedanimals and plants. As examples of this we findindications of careful experiments on seedling plants andweeds, to determine what proportion of them were destroyedby enemies before they came to maturity while another set;of observations determined the influence of the more robustin killing out the weaker plants with which they come intocompetition. This last fact, so simple in itself, yet so muchoverlooked, affords an explanation of many of the eccentricitiesof plant distribution, cultivation, and naturalisation.Every one who has tried it knows the difficulty or impossibilityof getting foreign plants, however hardy, to take careof themselves in a garden as in a state of nature. Whereverwe go among the woods, mountains, and meadows of thetemperate zone, we find a variety of charming flowers growingluxuriantly amid a dense vegetation of other plants, none ofwhich seem to interfere with each other. By far the largernumber of these plants will grow with equal luxuriance in