The surface of the earth being partly formed by water and partly by land, and the organization of all living beings standing in close relation to the one or the other of these mediums, it is in the nature of things that no single species, either of animals or plants, should be uniformly distributed over the whole globe. Yet there are some types of the animal as well as of the vegetable kingdom which are equably distributed over the whole surface of the land, and others which are as widely scattered in the sea, while others are limited to some continent or some ocean, to some particular province, to some lake, nay, to some very limited spot of the earth' surface.

As far as the primary divisions of animals are concerned, and the nature of the medium to which they are adapted does not interfere, representatives of the four great branches of the animal kingdom are everywhere found together. Radiata, Mollusks, Articulata, and Vertebrata occur together in every part of the ocean, in the Arctics, as well as under the equator, and near the southern pole as far as man has penetrated; every bay, every inlet, every shoal is haunted by them. So universal is this association, not only at present but in all past geological ages, that I consider it as a sufficient reason to expect that fishes will be found in those few fossiliferous beds of the Silurian System in which thus far they have not yet been found.