48 THE EVOLUTION THEORY 



small, of being suffocated by the surrounding vegetation, and so on. 

 We can thus understand, to some extent, though only approximately, 

 why it is that the oak must year by year produce thousands of seeds 

 in order that the species may maintain its normal number, and not be 

 exterminated; for it is obvious that a constant, even though slow 

 diminution of the normal number, a regular deficit, so to speak, can 

 end in nothing else than the gradual extinction of the species. 



But even this prodigality of seeds is not the greatest reach of 

 fertility that we meet with in nature : it is, perhaps, amongst the 

 simpler nowerless plants that we find the climax. It has been 

 calculated that a single frond of the beautiful fern so common in our 

 woods, Aspidiwni filix mas, produces about fourteen million spores. 

 They serve to distribute the species, and are carried as motes by the 

 wind, but comparatively few of the millions ever get the length of 

 germinating at all, much less of attaining to full development into 

 adult plants. Thus we see that the apparent prodigality of nature is 

 a real necessity, an indispensable condition of the maintenance of the 

 species; the fertility of each species is related to the actualities of 

 elimination to which it is exposed. This is clearly seen when 

 a species is placed under new and more favourable conditions of life, 

 in which it has an abundant food-supply and few enemies. This 

 was the case, for instance, with the horses introduced from Europe 

 into South America, where they reverted to a feral state, and are now 

 represented by herds of many thousands roaming the great grassy 

 plains. If the small singing-birds of a region diminish in number, 

 there is a great increase of caterpillars and other injurious insects 

 which form part of their food-supply. The colossal destruction 

 which the much-dreaded nun-moth from time to time brings about 

 in our woods probably depends in part on the diminution of 

 one or another of the many animals inimical to insects : but the 

 occurrence of several years of weather- conditions favourable to the 

 larvae must also be taken into account. How enormously, indeed 

 almost inconceivably, the number of larvae may increase under 

 favourable conditions is shown by such devastations as that in 

 Prussia in 1856, when many square miles of forest were absolutely 

 eaten up. The caterpillars were so numerous that even from some 

 distance the falling excrement could be heard rustling like rain, and 

 ten hundredweights of the eggs were collected, with an average of 

 20,000 eggs to the half-ounce ! 



But it would be a great mistake to conclude, from this enormous 

 and sudden increase in the number of individuals, that the normal 

 number of individuals is determined by the number of enemies alone. 



