Lectures on the Elements of Comparative Anatomy, 1864, p. 1.

Essai sur la Philosophie des Sciences, p. 9.

Lectures on the Elements of Comparative Anatomy, and on the Classification of Animals, 1864, p. 3.

Ossemens Fossiles, 4th edit. vol. i. p. 164. Quoted by Huxley, Lectures, &c., p. 5.

Chambers, Descriptive Astronomy, 1st edit. p. 23.

Philosophical Magazine, 4th Series, vol. xxxix. p. 396; vol. xl. p. 183; vol. xli. p. 44. See also Proctor, Popular Science Review, October 1874, p. 350.

Humboldt, Cosmos (Bohn), vol. iii. p. 224.