There are some seeming exceptions to these general statements respecting plant-distribution, but they are hardly exceptions when we consider the elevation at which any one species, as the birches for instance, may appear, as they frequently do, in three several zones. From these facts, gathered from the highest authorities, and well-attested on all hands, what general conclusions, if any, are to be drawn? Before answering this inquiry, let us proceed to state what conclusions _have_ been drawn. According to all the authorities we have examined on the distribution of plant life; on the migration of plants and animals; on climate and time as affecting the transference of isothermal and isochimenal lines; on glacial and inter-glacial periods (with one important exception only), the assumption maintained is substantially that of Mr. Renfrey, that "each species of plant and animal was created but once in time and place," and that its present diffusion is the result of its "own law of reproduction under the favorable or restrictive influences of laws external to it." In other words, they insist upon original plant-centres, without definitely stating when or where they occurred, and that from these centres both plants and animals have migrated to all parts of the globe where they now appear, even crossing the equatorial zones where they could not live for a single day. This migration theory they attempt to explain in a way that is altogether more ingenious than satisfactory. The important exception to which we refer is that of Professor Agassiz, as reported by his associate professor of Harvard University, Mr. Asa Gray, in his "Essays and Reviews Pertaining to Darwinism." In this work Professor Gray says of his late distinguished associate, that so far as he was aware, Professor Agassiz was the only leading naturalist "who did not take into his very conception of a species, explicitly or by implication, the notion of a material connection resulting from the descent of the individuals composing it from a common stock, of a local origin." And Professor Gray adds this further testimony to the closeness of his associate's observations, in considering the very point here under consideration: "Agassiz wholly eliminates community of descent from his idea of species, and even conceives a species to have been as numerous in individuals, and as widely spread over space, or as segregated in discontinuous spaces, from the first to the later periods." And this view is undoubtedly the correct one. At all events, it entirely harmonizes with the facts of the biblical genesis, and obviates the necessity of accounting for the appearance of the same genera and species of plants or animals in the southern as in the northern hemispheres; in fact, their appearance in all parts of the globe, in corresponding isotherms, and under similar conditions of moisture and soil-constituents.