PART II

§§ iv. and v. On the evidence from Geology. (The reasons for combining the two sections are given in the Introduction) 22

§ vi. Geographical distribution 29

§ vii. Affinities and classification 35

§ viii. Unity of type in the great classes 38

§ ix. Abortive organs 45

§ x. Recapitulation and conclusion 48

ESSAY OF 1844