Natural selection can easily be illustrated, and observed, in the case of
characteristics within a single species. Suppose that there is a population of
moths, some happening to be dark and others happening to be pale, who live
on birch trees and are preyed upon by birds. While the trees retain their
natural silver colour, the better-camouflaged pale moths will have a better
chance of survival, and will therefore come to form the greater part of the
population. If, however, the trees become blackened with soot, the odds of
survival will tilt in favour of the dark moths. As they survive in more than
average numbers, it will appear from the outside that the species is changing
its colour, from being characteristically pale to being characteristically dark.
Darwin believed that over a long period of time natural selection could go
further and create whole new species of plants and animals. This would,
indeed, be a process so slow as to be in the normal sense unobservable; but
recent discoveries in geology made plausible the idea that the earth had
existed for a sufficient length of time for species to come into and go out of
existence in this manner. Evolution could thus explain not only the like-
nesses and differences between existing species, but also the difference
between the species now extant and defunct species from earlier ages that
were being discovered in fossil form throughout the world. Even the most
complex organs and instincts, Darwin claimed, could be explained by the
accumulation of innumerable slight variations, each good for the individual.
To suppose that the eye, with all its inimitable contrivances for adjusting the focus
to different distances, for admitting different amounts of light, and for the
correction of spherical and chromatic aberration, could have been formed by
natural selection, seems, I freely confess, absurd in the highest possible degree. Yet
reason tells me, that if numerous gradations from a perfect and complex eye to one
very imperfect and simple, each grade being useful to its possessor, can be shown
to exist; if further, the eye does vary ever so slightly, and the variations be inherited,
which is certainly the case; and if any variation of modification in the organ be ever
useful to an animal under changing conditions of life, then the difficulty of
believing that a perfect and complex eye could be formed by natural selection,
though insuperable by our imagination, can hardly be considered real. (OS 152)
The case for Darwin’s theory was greatly strengthened after his death, first
when the laws of population genetics established by Gregor Mendel
became generally known, and then when the identification of DNA
enabled molecular geneticists to elucidate the mechanisms of heredity.
The story of Darwinism belongs to the history of science, not the history of
BENTHAM TO NIETZSCHE
27