Thus we see that the capacity for boundless multiplication inherent in every species is limited by the co-existence of other species; there is, metaphorically speaking, a continuous struggle going on between species, plant and animal alike; each seeks as far as possible to multiply, and each is hemmed in by the others and as far as possible prevented from doing so. The 'struggle' is by no means only the _direct_ limitation of the number of individuals, which consists in the use of one species by another as food, as in beasts of prey and their victims, or locusts and plants; it is much more the indirect limitation--figuratively speaking, the struggle for space, for light, for moisture among plants, for food among animals. But all this, important as it is, does not yet exhaust the content of that 'struggle for existence' to which Darwin and Wallace ascribe the rôle of the breeder in the process of natural selection. The struggle, that is, the mutual limiting of species, may indeed restrict a species in its distribution, and may reduce its normal number possibly to nil. In other words, it may bring about extinction, but it cannot make a species other than it is. This can only be done by a struggle within the limits of the species itself, and this struggle is due to the fact that of the numerous offspring, on an average those survive--that is, attain to reproduction--which are the most fit, whose constitution makes it most possible for them to overcome the difficulties and dangers of life, and so to reach maturity. We see, in fact, that a large percentage of each generation in all species always perishes before attaining maturity. If, then, the decision as to which is to perish and which is to reach maturity is _not a matter of chance alone_, but is in part due to the constitution of the growing individual; if the 'fittest' do _on the average_ survive, and the 'least fit' are on the average eliminated, we have here a process of selection entirely comparable to that of artificial selection, and one whose result must be the 'improvement' of the species, whether that depends on one set of characters or on another. The victorious qualities, which earlier were peculiar to certain individuals, must gradually become the common property of the species, if in each generation the individuals which attained to reproduction all possessed them, and thus could transmit them to their progeny. But those of the descendants which did not inherit them would again be at a disadvantage in the struggle for existence, or rather for reaching maturity, if in each generation a higher percentage of individuals which possess these characters reach maturity than of those which do not possess them. This percentage must increase in each generation, because, in each, natural selection again chooses out the fittest, and it must finally rise to 100 per cent., that is to say, none but individuals of this fittest type will be left surviving.