The tendency of organic life, whether vegetable or animal, being to propagate itself enormously,--and the life-sustaining capacity of the earth being limited,--it necessarily follows that only a fraction of the creatures which are born can survive to maturity, and that while those best fitted to live will live, those less well fitted will die. Thus, there is set up a constant struggle for existence, in which every advantage, however slight, must tell, so that those possessing such advantages in one generation will be the parents of the next. But in the course of propagation, the offspring never exactly reproduce the parent form, from which they vary, some in one way some in another, and as some of these variations cannot help being advantageous to their possessors in the struggle, we have here the required factor for the production of new forms. Any thus beneficially equipped, (although the variation, and consequently the advantage, must in each instance be exceedingly slight,) will have the chances on their side against their less favoured fellows, whom in the long run they will supplant. And as their offspring, or some of them, will carry the profitable variation somewhat further, the stream of life will thus be set in such a direction as will ultimately bring about what might at first appear impossible metamorphoses. Thus, to take a simple and favourite illustration,[180] winged insects inhabiting an island far from other land, are liable to be blown out to sea and drowned. It is in consequence, an advantage to them to have their power of flight curtailed, or taken away, and consequently in such situations their wings are generally found to be so reduced as to permit little or even nothing in the way of flying. Or to take an example of another kind,[181] the extraordinary length of neck which characterizes the giraffe enables it to browse on the higher branches of trees inaccessible to other vegetable feeders, and thus gives it an advantage over them in times of drought and scarcity of fodder. It can accordingly be easily understood, how its present structure has resulted from gradual elongations of the neck, each conferring on its possessor a slight advantage.