Indiscriminate Separation, under the Same Environment, a Cause of Divergence

Abstract

I HAVE accumulated a large body of facts indicating that separated fragments of a species, though exposed to the same environment, will in time become divergent. I find that, wherever a species possessing very low powers of migration is for many generations divided into a series of fragments by barriers that do not obstruct the distribution of surrounding species, more or less divergence arises in the separated portions of the species, though, in the same areas, there is no divergence in the environing species whose distribution is not obstructed. I still further find that, whenever the distances intervening between the different fragments are an approximate measure of the time and degree of separate breeding (as is frequently the case, as long as the divergence does not involve any physiological and psychological segregation), these distances are also an approximate measure of the degree of divergence.