But the germination of every seed depends on conditions _ab extra_, and all germs are modified, in their development, by geographical and climatal surroundings. The development of the acorn into a mature and perfect oak greatly depends on the exterior conditions of soil, and moisture, light, and heat. By these it may be rendered luxuriant in its growth, or it may be stunted in its growth. It may barely exist under one class of conditions, or it may perish under another. The Brassica oleracea, in its native habitat on the shore of the sea, is a bitter plant with wavy sea-green leaves; in the cultivated garden it is the cauliflower. The single rose, under altered conditions, becomes a double rose; and creepers rear their stalks and stand erect. Plants, which in a cold climate are annuals, become perennial when transported to the torrid zone.[6] And so human nature, fundamentally the same under all circumstances, may be greatly modified, both physically and mentally, by geographical, social, and political conditions. The corporeal nature of man--his complexion, his physiognomy, his stature; the intellectual nature of man--his religious, ethical, and esthetical ideas are all modified by his surroundings. These modifications, of which all men dwelling in the same geographical regions, and under the same social and political institutions, partake, constitute the _individuality_ of nations. Thus, whilst there is a fundamental basis of unity in the corporeal and spiritual nature of man, the causes of diversity are to be sought in the circumstances in which tribes and nations are placed in the overruling providence of God. [Footnote 6: See Carpenter's "Compar. Physiology," p. 625; Lyell's "Principles of Geology," pp. 588, 589.]