234 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
extraordinary fact if no variations had ever occurred useful to each 
being’s own welfare, in the same manner as so many variations have 
occurred useful to man. But if variations useful to any organic being 
ever do occur, assuredly individuals thus characterized will have the 
best chance of being preserved in the struggle for life; and from the 
strong principle of inheritance, these will tend to produce offspring 
similarly characterized. This principle of preservation, or the survival 
of the fittest, I have called Natural Selection. It leads to the im- 
provement of each creature in relation to its organic and inorganic 
conditions of life; and consequently, in most cases, to what must be 
regarded as an advance in organization. Nevertheless, low and simple 
forms will long endure if well fitted for their simple conditions of life. 
Natural selection, on the principle of qualities being inherited at 
‘corresponding ages, can modify the egg, seed, or young, as easily as 
the adult. Amongst many animals, sexual selection will have given 
its aid to ordinary selection, by assuring to the most vigorous and best 
adapted males the greatest number of offspring. Sexual selection will 
also give characters useful to the males alone, in their struggles or 
rivalry with other males; and these characters will be transmitted to 
one sex or to both sexes, according to the form of inheritance which 
preyails. 
Whether natural selection has really thus acted in adapting the 
various forms of life to their several conditions and stations, must be 
judged by the general tenor and balance of evidence given in the follow- 
ing chapters. But we have already seen how it entails extinction; and 
how largely extinction has acted in the world’s history, geology plainly 
declares. Natural selection, also, leads to divergence of character; 
for the more organic beings diverge in structure, habits, and constitu- 
tion, by so much the more can a large number be supported on the 
area, of which we see proof by looking to the inhabitants of any 
small spot, and to the productions naturalizedin foreignlands. There- 
fore, during the modification of the descendants of any one species, 
and during the incessant struggle of all species to increase in number, 
the more diversified the descendants become, the better will be their 
chance of success in the battle for life. Thus the small differences dis- 
tinguishing varieties of the same species, steadily tend to increase, till 
they equal the greater differences between species of the same genus, 
or even of distinct genera. 
We have seen that it is the common, the widely-diffused and 
widely ranging species, belonging to the larger genera within each 
