The question respecting the origin of animals and plants has likewise undergone a similar flux of beliefs, but with different result. Whatever the conclusions may be with regard to the origin of man, naturalists of the present day very generally agree, that there was no one universal centre of propagation for plants and animals; but that the same conditions of soil, moisture, heat, and geographical situation, always produce a similarity of species; or, what is equivalent, that there were many primary centres, each originating species, which spread out from these centres and covered the earth. This doctrine was held by early naturalists to be irreconcilable with the Scripture account of the creation, and was therefore denounced as heretical. Linnæus and his contemporaries drew up a pleasing picture, assigning the birth-place of all forms of life to one particular fertile spot, situated in a genial climate, and so diversified with lofty mountains and declivities, as to present all the various temperatures requisite for the sustenance of the different species of animal and vegetable life. The most exuberant types of flora and fauna are found within the tropical regions, decreasing in richness and profusion towards either pole; while man in his greatest perfection occupies the temperate zone, degenerating in harmony of features, in physical symmetry, and in intellectual vigor in either direction. Within this temperate zone is placed the hypothetical cradle of the human race, varying in locality according to religion and tradition. The Caucasians are referred for their origin to Mount Caucasus, the Mongolians to Mount Altai, and the Africans to Mount Atlas. Three primordial centres of population have been assigned to the three sons of Noah,--Arabia, the Semitic; India, the Japetic; and Egypt, the Hamitic centre. Thibet, and the mountains surrounding the Gobi desert, have been designated as the point from which a general distribution was made; while the sacred writings mention four rich and beautiful valleys, two of which are watered by the Tigris and Euphrates, as the birth-place of man. It was formerly believed that in the beginning, the primeval ocean covered the remaining portion of the globe, and that from this central spot the waters receded, thereby extending the limits of terrestrial life. Admitting the unity of origin, conjecture points with apparent reason to the regions of Armenia and of Iran, in western Asia, as the cradle of the human race. Departing from this geographical centre, in the directions of the extremities of the continent, the race at first degenerated in proportion to distance. Civilization was for many ages confined within these central limits, until by slow degrees, paths were marked out to the eastward and to the westward, terminating the one upon the eastern coast of Asia, and the other upon the American shores of the Pacific. [Sidenote: PRIMORDIAL CENTERS.]