El autor anuncia su intención de tratar con cierto detenimiento acerca de los medios de dispersión. Para comenzar, unas breves nociones de geología.       637 Before discussing the three classes of facts, which I have selected as presenting the greatest amount of difficulty on the theory of «single centres of creation,» I must say a few words on the means of dispersal. MEANS OF DISPERSAL.   Sir C. Lyell and other authors have ably treated this subject. I…