On the Origin of Species

by Charles Darwin

I am fully convinced that species are not immutable; but that those belonging to what are called the same genera are lineal descendants of some other and generally extinct species, in the same manner as the acknowledged varieties of any one species are the descendants of that species.

I may add, that when under nature the conditions of life do change, variations and reversions of character probably do occur; but natural selection, as will hereafter be explained, will determine how far the new characters thus arising shall be preserved.

I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection,

humble-bees

This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection.

I do believe that natural selection will always act very slowly, often only at long intervals of time, and generally on only a very few of the inhabitants of the same region at the same time. I further believe, that this very slow, intermittent action of natural selection accords perfectly well with what geology tells us of the rate and manner at which the inhabitants of this world have changed.

We should be extremely cautious in concluding that an organ could not have been formed by transitional gradations of some kind.