In this same year, 1853 , Dr. Schaaffhausen published an excellent pamphlet ( Verhand. des Naturhist. Vereins der Preuss. Rheinlands , etc. ), in which he maintains the development of organic forms on the earth. He infers that many species have kept true for long periods, whereas a few have become modified. The distinction of species he explains by the destruction of intermediate graduated forms. “Thus living plants and animals are not separated from the extinct by new creations, but are to be regarded as their descendants through continued reproduction.”