[10] Hugh Robert Mill, International Geography, p. 347. New York, 1902. [11] Joseph Partsch, Central Europe, pp. 228-230. London, 1903. [12] H.J. Mackinder, Britain and the British Seas, pp. 317-323. London, 1904. [13] Captain A.T. Mahan, Influence of Sea Power upon History, pp. 36-38. Boston, 1902. [14] G.G. Chisholm, Economic Geography, _Scottish Geog. Mag_., March, 1908. [15] Captain A.T. Mahan, Influence of Sea Power upon History, pp. 37-38. Boston, 1902. [16] Boyd Winchester, The Swiss Republic, pp. 123, 124, 145-147. Philadelphia, 1891. [17] Montesquieu, Spirit of the Laws, Book XIV, chap. IV. [18] Henry Buckle, History of Civilization in England, Vol. I, pp. 86-106. [19] Heinrich von Treitschke, _Politik_, Vol. I, p. 225. Leipzig, 1897. This whole chapter on _Land und Leute_ is suggestive. [20] W.Z. Ripley, Races of Europe, pp. 524-525. New York, 1899. [21] _Ibid._, 526. [22] _Ibid._, 517-520, 533-536. [23] Joseph Partsch, Central Europe, pp. 256-257, 268-271. London, 1903. [24] W.Z. Ripley, Races of Europe, p. 89. New York, 1899. [25] Strabo, Book VII, chap. I, 2. [26] Strabo, Book II, chap. III, 7. [27] Plutarch, Solon, pp. 13, 29, 154. [28] Hans Helmolt, History of the World, Vol. II, pp. 244-245. New York, 1902-1906. [29] Roscher, _National-oekonomik des Ackerbaues_, p. 33, note 3. Stuttgart, 1888. [30] Captain A.T. Mahan, Influence of Sea Power upon History, pp. 41-42, 50-53. Boston, 1902. [31] H. Bancroft, History of California, Vol. I, pp. 298, 628-635. San Francisco. [32] Agnes Laut, Vikings of the Pacific, pp. 64-82. New York, 1905. [33] Bunbury, History of Ancient Geography, Vol. II, pp. 351, 470-471. London, 1883. CHAPTER II CLASSES OF GEOGRAPHIC INFLUENCES Into almost every anthropo-geographical problem the element of environment enters in different phases, with different modes of operation and varying degrees of importance. Since the causal conception of geography demands a detailed analysis of all the relations between environment and human development, it is advisable to distinguish the various classes of geographic influences. [Sidenote: Physical effects.] Four fundamental classes of effects can be distinguished. 1. The first class includes direct physical effects of environment, similar to those exerted on plants and animals by their habitat. Certain geographic conditions, more conspicuously those of climate, apply certain stimuli to which man, like the lower animals, responds by an adaption of his organism to his environment. Many physiological peculiarities of man are due to physical effects of environment, which doubtless operated very strongly in the earliest stages of human development, and in those shadowy ages contributed to the differentiation of races. The unity of the human species is as clearly established as the diversity of races and peoples, whose divergences must be interpreted chiefly as modifications in response to various habitats in long periods of time.