It is well known that the fertility of “virgin” soils is due to the accumulation of the débris of a natural vegetation which has been in occupation of the soil for a long epoch previously. Only when the climate and rainfall are suitable to the growth of the plants and the partial preservation of their residues does a virgin soil of any richness arise; on the one hand, virgin soil may be as poverty stricken as the most worn-out European field because it has never carried any vegetation; on the other hand, as in the tropics, the débris of an extensive vegetation may decay with such rapidity that no reserve of fertility accumulates.