The natural sciences are connected by the same ties that link all natural phenomena together. The classification of species, which we should consider as fundamental to botany, and whose study has been facilitated by introducing natural methods, is to plant geography what descriptive mineralogy is to the rocks that form the outer crust of the earth. To understand the laws observed in the rocks, and to determine the age of successive formations and identify them from the most distant regions, a geologist should know the simple fossils that make up the mass of mountains. The same goes for the natural history that deals with how plants are related to each other, and with the soil and air. The advancement of plant geography depends greatly on descriptive botany; it would hinder the advancement of the sciences to postulate general ideas by neglecting particular facts.