ILLUSTRATIONS (13). HUM EBICAL DISTRIBUTION. 287 



hypothesis of migrations of plants radiating from certain 

 central points. Thermal relations are scarcely sufficient to 

 explain the phenomenon why certain species have fixed limits 

 beyond which they cannot pass, either in the plains towards 

 the pole, or in vertical elevation on the declivities of moun- 

 tains. The cycle of vegetation of each species, however 

 different may be its duration, requires a certain minimum 

 of temperature to enable it to arrive at the full stage of its 

 development.* But all the conditions necessary to the 

 existence of a plant, either within its natural sphere of dis- 

 tsibution or cultivation such as geographical distance from 

 the pole, and elevation of the locality are rendered still 

 more complicated by the difficulty of determining the begin- 

 ning of the thermic cycle of vegetation; by the influence 

 which the unequal distribution of the same quantity of heat 

 among days and nights succeeding each other in groups, 

 exerts on the irritability, the progressive development, and 

 the whole vital process ; and lastly, by the secondary influence 

 of the hygrometric and electric relations of the atmosphere. 



My investigations regarding the numerical laws of the dis- 

 tribution of vegetable forms may, perhaps, at some future time, 

 be applied successfully to the different classes of verte- 

 brate animals. The rich collections of the Museum d'histoire 

 naturelle in the Jardin des Plantes at Paris, contained in 1820, 

 at a rough estimate, above 56,000 species of phanerogamic and 

 cryptogamic plants in the herbariums, 44,000 insects (proba- 

 bly below the actual number, although they were thus given 

 me by Latreille), 2500 species of fishes, 700 reptiles, 4000 birds, 

 and 500 mammalia. Europe possesses about 80 mammalia, 

 400 birds, and 30 reptiles; there are, therefore, five times as 

 many birds as mammalia in the northern temperate zone, (as 

 there are in Europe five times as many Composite as Amenta- 

 cea3 and Coniferae, and five times as many Leguminosae as 

 Orchideae and Euphorbiaceae). In the southern temperate 

 zone the ratio of the Mammalia bears a sufficiently striking 

 accord with that of Birds, being as 1 : 4'3. Birds (and rep- 



* Playfair, in the Transactions of the Royal Soc,. of Edirib., vol. v. 

 1805, p. 202; Humboldt, on the sum total of the thermometric degrees 

 required for the cycle of vegetation of the Cereals, in Mem. sur des 

 lignes isotliermes, p. 96; Boussingault, Economic rurale, t. ii. p. 659, 

 663, 667; and Alphonee Decandolle, Sur les causes qui limitent lei 

 eepeces vegetales, 1847, p. 8. 



