As I have repeatedly said, but cannot too much emphasize, the actual value and invincible strength of the Theory of Descent does not lie in its explaining this or that single phenomenon, but in the fact that it explains _all_ biological phenomena, that it makes _all_ botanical and zoological series of phenomena intelligible in their relations to one another. Hence every thoughtful investigator is the more firmly and deeply convinced of its truth the more he advances from single biological observations to a general view of the whole domain of animal and vegetable life. Let us now, starting from this comprehensive point of view, survey a biological domain, the varied and complicated phenomena of which may be explained with remarkable simplicity and clearness by the theory of selection. I mean _Chorology_, or the theory of the _local distribution of organisms over the surface of the earth_. By this I do not only mean the _geographical_ distribution of animal and vegetable species over the different parts and provinces of the earth, over continents and islands, seas, and rivers; but also their _topographical_ distribution in a _vertical_ direction, their ascending to the heights of mountains, and their descending into the depths of the ocean. (Gen. Morph. ii. 286.) The strange chorological series of phenomena which show the horizontal distribution of organisms over parts of the earth, and their vertical distribution in heights and depths, have long since excited general interest. In recent times Alexander Humboldt(39) and Frederick Schouw have especially discussed the geography of plants, and Berghaus and Schmarda the geography of animals, on a large scale. But although these and several other naturalists have in many ways increased our knowledge of the distribution of animal and vegetable forms, and laid open to us a new domain of science, full of wonderful and interesting phenomena, yet Chorology as a whole remained, as far as their labours were concerned, only a desultory knowledge of a mass of individual _facts_. It could not be called a science as long as the _causes_ for the explanation of these facts were wanting. These causes were first disclosed by the theory of selection and its doctrine of the _migrations_ of animal and vegetable species, and it is only since the works of Darwin and Wallace that we have been able to speak of an independent _science of Chorology_.