It is easy to understand that these prevalent ideas of the absolute unchangeability and supernatural creation of organic species could not satisfy the more penetrating thinkers. We find several eminent minds already, in the second half of the last century, busy with the attempt to find a natural explanation of the "problem of creation." Pre-eminent among them was the great German poet and philosopher, Wolfgang Goethe, who, by his long and assiduous study of morphology, obtained, more than a hundred years ago, a clear insight into the intimate connection of all organic forms, and a firm conviction of a common natural origin. In his famed _Metamorphosis of Plants_ (1790) he derived all the different species of plants from one primitive type, and all their different organs from one primitive organ--the leaf. In his vertebral theory of the skull he endeavored to prove that the skulls of the vertebrates--including man--were all alike made up of certain groups of bones, arranged in a definite structure, and that these bones are nothing else than transformed vertebræ. It was his penetrating study of comparative osteology that led Goethe to a firm conviction of the unity of the animal organization; he had recognized that the human skeleton is framed on the same fundamental type as that of all other vertebrates--"built on a primitive plan that only deviates more or less to one side or other in its very constant features, and still develops and refashions itself daily." This remodelling, or transformation, is brought about, according to Goethe, by the constant interaction of two powerful constructive forces--a centripetal force within the organism, the "tendency to specification," and a centrifugal force without, the tendency to variation, or the "idea of metamorphosis"; the former corresponds to what we now call heredity, the latter to the modern idea of adaptation. How deeply Goethe had penetrated into their character by these philosophic studies of the "construction and reconstruction of organic natures," and how far, therefore, he must be considered the most important precursor of Darwin and Lamarck,[12] may be gathered from the interesting passages from his works which I have collected in the fourth chapter of my _Natural History of Creation_. These evolutionary ideas of Goethe, however, like analogous ideas of Kant, Owen, Treviranus, and other philosophers of the commencement of the century (which we have quoted in the above work), did not amount to more than certain general conclusions. They had not that great lever which the "natural history of creation" needed for its firm foundation on a criticism of the dogma of fixed species; this lever was first supplied by Lamarck.