If we do not accept the hypothesis of spontaneous generation, then at this one point of the history of development we must have recourse to the miracle of a _supernatural creation_. The Creator must have created the first organism, or a few first organisms, from which all others are derived, and as such he must have created the simplest Monera, or primæval cytods, and given them the capability of developing further in a mechanical way. I leave it to each one of my readers to choose between this idea and the hypothesis of spontaneous generation. To me the idea that the Creator should have in this one point arbitrarily interfered with the regular process of development of matter, which in all other cases proceeds entirely without his interposition, seems to be just as unsatisfactory to a believing mind as to a scientific intellect. If, on the other hand, we assume the hypothesis of spontaneous generation for the origin of the first organisms, which in consequence of reasons mentioned above, and especially in consequence of the discovery of the Monera, has lost its former difficulty, then we arrive at the establishment of an uninterrupted natural connection between the development of the earth and the organisms produced on it, and, in this last remaining lurking-place of obscurity, we can proclaim the _unity of all Nature, and the unity of her laws of Development_ (Gen. Morph. i. 164). CHAPTER XIV. MIGRATION AND DISTRIBUTION OF ORGANISMS. CHOROLOGY AND THE ICE-PERIOD OF THE EARTH. Chorological Facts and Causes.—Origin of most Species in one Single Locality: “Centres of Creation.”—Distribution by Migration.—Active and Passive Migrations of Animals and Plants.—Means of Transport.—Transport of Germs by Water and by Wind.—Continual Change of the Area of Distribution by Elevations and Depressions of the Ground.—Chorological Importance of Geological Processes.—Influence of the Change of Climate.—Ice or Glacial Period.—Its Importance to Chorology.—Importance of Migrations for the Origin of New Species.—Isolation of Colonists.—Wagner’s Law of Migration.—Connection between the Theory of Migration and the Theory of Selection.—Agreement of its Results with the Theory of Descent.