52 THE EVOLUTION THEORY 



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 role 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 dis- 

 tribution, 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 elm ace alone r 

 but is in part due to the constitution of the growing individual ; if 

 the ' fittest ' do on the average survive, and the c least fit ' are on the 

 average eliminated, we have here a process of selection entirely com- 

 parable 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 ep.rlier 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 ico per 



