110.
[18] Ibid.p.

111.
[19] Ibid.p.

227.
[20] The order _Ungulata_ contains the hoofed beasts; that is, all oxen, deer, antelopes, sheep, goats, camels, hogs, the hippopotamus, the different kinds of rhinoceros, the tapirs, horses, asses, zebras, quaggas, &c.
[21] The elephants of Africa and India, with their extinct allies, constitute the order _Proboscidea_, and do not belong to the Ungulata.
[22] See "Natural Selection," pp.

60-75.
[23] "Principles of Biology," vol.i.p.

122.
[24] See "Natural Selection," chap.iii.p.