Link, _Die Urwelt und das Altertum_, vol. i, p. 84. Footnote 86: Link, _op. cit._, vol. i, p. 91. Footnote 87: Cuvier, _op. cit._ Compare also, on this point, the opinion of Alexander von Humboldt: “In the epochs preceding the existence of the human race the action of the forces in the interior of the globe must, as the earth’s crust increased in thickness, have modified the temperature of the air and made the whole earth habitable by the products which we now regard as exclusively tropical. Afterwards the spatial relation of our planet to the central body (the sun) began, by means of radiation and cooling down, to be almost the sole agent in determining the climate at different latitudes. It was also in these primitive times that the elastic fluids, or volcanic forces, inside the earth, more powerful than they are to-day, made their way through the oxidized and imperfectly solidified crust of our planet” (_Asie centrale_, vol. i, p. 47). Footnote 88: Second edition, pp. 92–4. The man was born in 1727. Footnote 89: _See_ Genesis ii, 8, 10, 15. Footnote 90: Lyell, “Principles of Geology,” vol. ii, p. 119. Footnote 91: Alexander von Humboldt does not think that this hypothesis can apply to the migration of plants. “What we know,” he says, “of the deleterious action exerted by sea-water, during a voyage of 500 or 600 leagues, over the reproductive power of most grains, does not favour the theory of the migration of vegetables by means of ocean currents. Such a theory is too general and comprehensive” (_Examen critique de l’histoire de la géographie du nouveau continent_, vol. ii, p. 78). Footnote 92: