[33] For examples of this the reader is referred to Cope, “Bulletin of the National Museum,” No. 1; and to Coues’s “Key to North American Birds,” last edition. [34] “Monatsbericht d. k. Preuss. Akademie d. Wissenschaft zu Berlin,” for July, 1866. [35] “Genesis of Tertiary Species of Planorbis at Steinheim.” A. Hyatt, Anniversary Memoir of the Boston Society of Natural History, 1880. [36] In a letter to the author, dated February 13, 1887, Prof. Cope says: “Such transitions of species are clearly indicated in the _Oreodontidæ_, where such different forms as _O. gracilis_ and _O. Culbertsoni_ are connected by intergradations.” [37] “American Naturalist,” 1873; “Popular Science Monthly,” June, 1873. [38] For a fuller development of this subject the reader is referred to an article by the author, entitled “Critical Periods in the History of, and their Relation to, Evolution” (“American Journal of Science,” vol. xiv, p. 99, 1877). [39] “Reflex Action and Theism,” William James, “Unitarian Review” for November, 1881. [40] See an article by the author on this subject, “American Journal of Science,” series ii, vol. xxviii, p. 305, 1859, and in “Popular Science Monthly,” vol. iv, p. 156, 1873. [41] All chemical compounds are dissociated by sufficient heat. [42] The origin of vital from chemical force in the green leaves of plants can not be doubted; but this does not, of course, explain the mystery of the _first origin of life on the earth_, for one condition of the change _now_ is the _contact of living matter_. [43] I know it is the fashion to ridicule the use of the terms vitality, vital force, as a remnant of an old superstition; and yet the same men who do so use the terms gravity, electricity, chemical force, etc. Vital force is indeed _correlated_ with other forces of Nature, but is none the less a distinct _form_ of force, far more distinct than any other unless it be the still higher form of psychical, and therefore it better deserves a distinct name than any lower form. Each form of force gives rise to a peculiar group of phenomena, and the study of these to a special department of science. Now, the group of phenomena called vital is more peculiar, more different from other groups than these are from each other, and the science of physiology is a more distinct department than either physics or chemistry, and therefore the form of force, which determines these phenomena, is more distinct and better entitled to a name than any physical or chemical force. [44] “Princeton Review” for May, 1884. [45] “Popular Science Monthly,” December, 1873. [46] For a fuller statement of this antithesis, see an article by the author entitled “Evolution in Relation to Materialism,” “Princeton Review,” for March, 1881. [47] Johnstone Stoney, “Nature,” vol. xxxi, p. 422. [48] “Nature,” vol. xxxiv, p. 385. 1886.