It was impossible to retreat simply to Linné’s position. Lyell by no means denied Cuvier’s various periods in the earth’s development as such. He believed, moreover, that the plant and animal populations were different in these epochs. When the forests flourished which have formed the mass of our coal-measures there were no ichthyosauri; when the ichthyosauri came there were no longer any carboniferous forests; with the ichthyosauri there were no megatheria, and the last ichthyosaurus was extinct before the megatheria arrived. All that Lyell rejected was the great divine catastrophes. But when these were abandoned, it was no longer possible to attribute the “end” of the extinct species to a divine act. We were faced with the slow and natural conversion of terrestrial things in the course of endless ages. Species must have been liable to be destroyed by purely natural causes. The catastrophes were abandoned, yet species _had_ been destroyed. And when that was granted—it was the devil’s little finger—a further conclusion was inevitable. If species have died out slowly and naturally in the history of the earth, and new species have made their appearance at the same time, may not these new species have _arisen_ slowly and naturally? Suppose these simple and purely natural causes, that had brought about the extinction of certain species, had been for others the very starting-point of development? In one word: if the extinction was not due to a mighty divine interference, was it not conceivable that the origin also may not have needed such? One more deduction, and the demon of knowledge had hold of the entire hand. May not this natural extinction and natural new-birth have been directly connected in many cases? As a fact, some of the species had been wholly extirpated. But others had provided the living material of the new arrivals; they had been _transformed_ into these apparently new species. That was the decisive deduction. It did away with the need of any sudden creation. It merely made a claim that was appalling to the Linnean principles: namely, that species may change. In the course of time and at a favourable spot one species may be transformed into another.