20 The Selectio7i Theory 



special features of the Darwinian conception of nature. That there 

 are carnivorous animals wliich take heavy toll in every generation of 

 the progeny of the animals on which they prey, and that there are 

 herbivores which decimate the plants in every generation had long 

 been known, but it is only since Darwin's time that sufficient at- 

 tention has been paid to the facts that, in addition to this regular 

 destruction, there exists between the members of a species a keen 

 competition for space and food, which limits multiplication, and that 

 numerous individuals of each species perish because of unfavourable 

 climatic conditions. The ''struggle for existence," which Darwin re- 

 garded as taking the place of the human breeder in free nature, is 

 not a direct struggle between carnivores and their prey, but is the 

 assumed competition for survival between individuals of the same 

 species, of >vhich, on an average, only those survive to reproduce 

 which have the greatest power of resistance, while the others, less 

 favourably constituted, perish early. This struggle is so keen, that, 

 within a limited area, where the conditions of life have long re- 

 mained unchanged, of every species, whatever be the degree of 

 fertility, only two, on an average, of the descendants of each pair 

 survive; the others succumb either to enemies, or to disadvantages 

 of climate, or to accident. A high degree of fertility is thus not an 

 indication of the special success of a species, but of the numerous 

 dangers that have attended its evolution. Of the six young brought 

 forth by a pair of elephants in the course of their lives only two 

 survive in a given area ; similarly, of the millions of eggs which two 

 thread-worms leave behind them only two survive. It is thus possible 

 to estimate tlie dangers which threaten a species by its ratio of 

 elimination, or, since this cannot be done directly, by its fertility. 



Although a gi-eat number of the descendants of each generation 

 fall victims to accident, among those that remain it is still the greater 

 or lesser fitness of the organism that determines the "selection for 

 breeding purposes," and it would be incomprehensible if, in this 

 competition, it Mere not ultimately, that is, on an average, the best 

 equipped Avhich survive, in the sense of living long enough to re- 

 I)roduce. 



Thus the principle of natural selection is the selection of the 

 lest for reproduction, whether the " best " refers to the whole con- 

 stitution, to one or more parts of the organism, or to one or more 

 stages of development. Every organ, every part, every character of 

 an anin)al, fertility and intelligence included, must be improved in 

 this manner, and be gradually brought up in the course of genera- 

 tions to its highest attainable state of perfection. And not only may 

 improvement of parts be brought about in this way, but new parts 

 and organs may arise, since, through the slow and minute steps of 



