20 The Selection Theory 
special features of the Darwinian conception of nature. That there 
are carnivorous animals which 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 which, 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 pot 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 the dangers which threaten a species by its ratio of 
elimination, or, since this cannot be done directly, by its fertility. 
Although a great 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 were not ultimately, that is, on an average, the best 
equipped which survive, in the sense of living long enough to re- 
produce. 
Thus the principle of natural selection is the selection of the. 
best 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 animal, 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 
