Thus, when varieties enter any new station, they occasionally assume some of the characters proper to the species of that station.

With both varieties and species, use and disuse seem to have produced a considerable effect; for it is impossible to resist this conclusion when we look, for instance, at the logger-headed duck, which has wings incapable of flight, in nearly the same condition as in the domestic duck; or when we look at the burrowing tucu-tucu, which is occasionally blind, and then at certain moles, which are habitually blind and have their eyes covered with skin; or when we look at the blind animals inhabiting the dark caves of America and Europe.

With varieties and species, correlated variation seems to have played an important part, so that when one part has been modified other parts have been necessarily modified.

With both varieties and species, reversions to long-lost characters occasionally occur.