ANNOTATIONS AND ADDITIONS. 113 



requires, in order to be successfully passed through, a cer- 

 tain minimum of temperature. (Playfair, in the Transac- 

 tions of the Koyal 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, Economic 

 rurale, T. ii. p. 659, 663, and 667 ; Alphonse Decandolle 

 sur les causes qui limitent les especes vegetales, 1*47, 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 Kotiferse in 

 the animal creation. The rich collections at the Museum 

 d'Histoire Naturelle in the Jardin des Plantes at Paris* 

 already contained, in 1820, (acccording to approximate esti- 

 mations) above 56000 phsenogamous and cryptogamous 

 plants in herbariums, 44000 insects (a number doubtless 

 too small, though given me by Latreille), 2500 species of 



VOL. II. I 



