Page:Aspects of nature in different lands and different climates; with scientific elucidations (IA b29329668 0002).pdf/121

 *[Footnote: requires, in order to be successfully passed through, a certain minimum of temperature. (Playfair, in the Transactions of the Royal Society of Edinburgh, vol. v. 1805, p. 202; Humboldt, on the sum of the degrees of temperature required for the cycle of vegetation in the Cerealia, in Mem. sur les lignes isothermes, p. 96; Boussingault, Economie rurale, T, ii. p. 659, 663, and 667; Alphonse Decandolle sur les causes qui limitent les espèces végétales, 1847, p. 8.) But all the conditions necessary for the existence of a plant, either as diffused naturally or by cultivation,—conditions of latitude or minimum distance from the pole, and of elevation or maximum height above the level of the sea,—are farther complicated by the difficulty of determining the commencement of the thermic cycle of vegetation, and by the influence which the unequal distribution of the same quantity of heat into groups of successive days and nights exercises on the excitability, the progressive development, and the whole vital process; to all this must be farther added hygrometric influences and those of atmospheric electricity.

My investigations respecting the numerical laws of the distribution of forms may possibly be applied at some future day with advantage to the different classes of Rotiferæ in the animal creation. The rich collections at the Museum d'Histoire Naturelle in the Jardin des Plantes at Paris, already contained, in 1820, (according to approximate estimations) above 56000 phænogamous and cryptogamous plants in herbariums, 44000 insects (a number doubtless too small, though given me by Latreille), 2500 species of]*