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Huxley had led many geologists to infer that any step in the successional series must have occurred simultaneously all over the earth, so that two series of rocks containing the same fossils were held to be of contemporaneous origin, however distant from one another they might be. Huxley gave a forcible summary of the evidence against this view, and declared that 'neither physical geology nor palaeontology possesses any method by which the absolute synchronism of two strata can be demonstrated. All that geology can prove is local order of succession.' The justice of this statement has not been questioned ; and the limitation imposed by it is one of the many difficulties encountered when we attempt to learn the ancestral history of animals from the fossil records.

In 1863 he delivered a course of lectures at the College of Surgeons 'On the Classification of Animals,' and another 'On the Vertebrate Skull.' These lectures were published together in 1864. Other courses 'On the Comparative Anatomy of Vertebrates' followed, and a condensed summary of these was published as a 'Manual of the Comparative Anatomy of Vertebrated Animals' in 1871. The scrupulous care with which he endeavoured to verify by actual observation every statement made in his lectures rendered the labour of preparation very great. Sir William Flower [q. v. Suppl.] describes the way in which he would spend long evenings at the College of Surgeons, dissecting animals available among the stores, or making rapid notes and drawings, after a day's work in Jermyn Street. The consequences were twofold ; the vivid impression of his own recent experience was communicated to his hearers, and the work of preparation became at once an incentive to further research and a means of pursuing it.

The lectures in 1867 dealt with birds, and Professor Newton writes of them : 'It is much to be regretted that his many engagements hindered him from publishing in its entirety his elucidation of the anatomy of the class, and the results which he drew from his investigations of it ; for never, assuredly, had the subject been attacked with greater skill and power, or, since the days of Buffon, had ornithology been set forth with greater eloquence' (, A Dictionary of Birds, p. 38). One great result of the work on birds, together with the study of fossil reptiles, was a recognition of the fundamental similarities between the two, which Huxley expressed by uniting birds and reptiles in one great group, the Sauropsida. Other results obtained were shortly summarised in an essay 'On the Classification of Birds' (Zool. Soc. Proc. 1867), containing an elaborate account of the modifications exhibited by the bones of the palate. This essay exhibits in an entirely new light the problems which have to be solved before we can establish a natural classification of birds. The solution offered has not been accepted as final ; but there is no question about the great value of the essay as a contribution to cranial morphology.

The lectures on birds must serve as examples of others given at the College of Surgeons; they were probably the most strikingly novel of any except the first course 'On the Classification of Animals ;' but the condensed summary, published in 1871, shows that every course of lectures must have marked important additions to our knowledge of the animals with which it dealt. One other important problem, that of the homologies of the bones which connect the tympanic membrane with the ear-capsule, must be mentioned as treated in these lectures, and more fully in a paper read before the Zoological Society (1869).

Apart from the lectures, and from the books based on them, Huxley published about fifty technical papers between 1860 and 1870. Among these are numerous descriptions of dinosauria, including that of hypsilophodon, the results being summarised in the essay on the classification of the group (Quart. Journ. Geol. Soc. 1869), and in the statements of the relation between reptiles and birds, already referred to. The account of hyperodapedon (1869) is of great importance in connection with another group of reptiles, and there are many valuable memoirs on fossil amphibia. Much of his work on systematic ethnology remains unpublished ; but in 1865 he published an essay 'On the Methods and Results of Ethnology,' containing a scheme of classification of the races of mankind, based on the characters of the hair, the colour of the skin, and the cranial index. He evidently contemplated a more complete study of physical anthropology ; for among the materials left in his laboratory are some hundreds of photographs of various races of men, which he had collected before 1870.

The 'Elementary Lessons in Physiology,' published in 1866, is probably better known than any elementary text-book of its kind. It has been reprinted no less than thirty times since its first appearance.

The years from 1870 to 1885 comprise a period of constant activity, ending in an almost complete withdrawal from public life, made necessary by increasing illness.

In 1872 the removal of the School of