623 far as we know them, are, firstly, circumstances hostile to the particular character in question ; secondly, conditions of life incessantly inducing fresh variability; and, lastly, the crossing of distinct varieties during some previous generation, together with reversion or atavism,-that is, the tendency of the child to resemble its grandparents or more remote ancestors, instead of its immediate parents. He discusses this subject of Reversion in the next chapter