down in a fashion quite different, and that it was impossible to regard both skull and backbone as modifications of a common type laid down right along the axis of the body. The spinal column and the skull start from the same primitive condition, whence they immediately begin to diverge. It may be true to say that there is a primitive identity of structure between the spinal or vertebral column and the skull; but it is no more true that the adult skull is a modified vertebral column than it would be to affirm that the vertebral column is a modified skull." Since this famous lecture, a number of distinguished anatomists have studied the development of the skull more fully; but they have not departed from the methods of investigation laid down by Huxley, and their conclusions have differed only in greater elaboration of detail from the broad lines laid down by him. Apart from its direct scientific value, this lecture was of importance as marking the place to which Huxley had attained in the scientific world. Two years later, it is true, the London _Times_, referring to a famous debate at a meeting of the British Association at Oxford, spoke of him as "a Mr. Huxley"; but in the scientific world he was accepted as the leader of the younger anatomists, and as one at least capable of rivalling Owen, who was then at the height of his fame. The Croonian Lecture was in a sense a deliberate challenge to Owen, and in these days before Darwin, to challenge Owen was to claim equality with the greatest name in anatomical science. CHAPTER V CREATURES OF THE PAST Beginning Palæontological Work--Fossil Amphibia and Reptilia--Ancestry of Birds--Ancestry of the Horse--Imperfect European Series Completed by Marsh's American Fossils--Meaning of Geological Contemporaneity--Uniformitarianism and Catastrophism Compared with Evolution in Geology--Age of the Earth--Intermediate and Linear Types.