The process of modification must be slow, and will generally affect only a few species at the same time; for the variability of each species is independent of that of all others.

Whether such variations or individual differences as may arise will be accumulated through natural selection in a greater or less degree, thus causing a greater or less amount of permanent modification, will depend on many complex contingencies--on the variations being of a beneficial nature, on the freedom of intercrossing, on the slowly changing physical conditions of the country, on the immigration of new colonists, and on the nature of the other inhabitants with which the varying species come into competition.

Hence it is by no means surprising that one species should retain the same identical form much longer than others; or, if changing, should change in a less degree.