impassableness, but which until recently has been left entirely out of account. This is the element of _time_--the amount of time since the barrier was set up, or during which it has continued to exist. These two elements, it is true, are closely connected with each other; for, since all changes in physical geography have taken place very slowly--since barriers in the form of mountain-ranges and seas have increased by slow process of growth--it is evident that impassableness is, to some extent, a measure of time. But they are by no means in strict proportion. The one or the other may predominate. Now, this time-element connects geographical distribution with changes of physical geography and climate in _geological_ times, and especially with the _latest_ of these changes, viz., those occurring during the _Glacial epoch_. During that remarkable epoch extraordinary changes of climate, from extreme Arctic rigor to great mildness, enforced wide migrations of species southward and northward; while concomitant changes of physical geography, by elevation of the earth’s crust over wide areas, opened highways between previously-isolated continents, permitting migrations in various directions, and by subsequent depression again isolating the migrated species in their new homes. It is evident, then, that the recognition of the element of almost unlimited time at once introduces into the question of geographical distribution the _idea of evolution_. If the study of geographical distribution, as _it now exists_, and as a part of science of physical geography, gave rise naturally to the theory of “specific centers of origin,” the study of the same, in connection with geological time, and as a part of geological science, now demands its explanation by the theory of evolution. It must be borne in mind, then, that geographical diversity of organisms is not a question of the present epoch only. There has been geographical diversity in every previous geological epoch; it is, therefore, a question of geology as well as of biology. It is probable, however, that diversity has increased with the course of geological times, and is greater now than ever before. In other words, in the evolution of the organic kingdom, the law of differentiation has prevailed here, as in other departments of biology. A clear statement of the causes of the _present_ distribution of organisms must embrace also the causes of geographical diversity _generally_. We give, therefore, at once a brief statement of what seems to us the most probable view, and shall then proceed to show how it explains the present distribution.