472 GEOGRAPHY OF PLANTS. 



CHAPTER II. 



ON THE INFLUENCE OF PHYSICAL AGENTS ON THE ORGANIC SERIES. 



Of the Geography of Plants: their horizontal and vertical Localization. Influence of Heat on or- 

 ganic Distribution : isotheral and isochimenal Conditions. Effects of Variations in the Dens- 

 ity of the Air, Moisture, Soil, Sunlight, Length of Day. Definite Quantity of Heat required 

 by Plants. 



Secular Perturbations in the Species of Plants. Long Periods of Time required. Secular geo- 

 logical Changes. 



Inverse Problem of the Investigation of the Earth's History from her fossil Flora. Two great 

 terrestrial Epochs : Change in the Constitution of the Air, and Localization of Organisms 

 through Decline of the Earth's Interior Heat. 



Difference between abrupt and gradual Impressions. Invariable Causes may produce abrupt 

 Crises. , 



Extension of the above Principles to the Case of Animals. Case of the Inca Indians. 



General Argument supported by the Extinction of Forms. Development is under the Influence of 

 Law. Rudimentary Organs and Excesses of Development. The Idea of Development by 

 Law consistent with natural Facts. 



THE publication of HUMBOLDT'S Essay on the Geography of Plants 

 Geo ra hical ^ rst f rma ^y drew the attention of botanists to the connec- 

 distribution of tion between the distribution of vegetables and the distribu- 

 tion of heat on the surface of the globe. Starting from the 

 equator and advancing to the pole, in either hemisphere, the mean annual 

 temperature declines as the latitude becomes greater, and in succession a 

 series of vegetable zones, merging gradually into each other, though each, 

 where best marked, perfectly distinguished from the succeeding, is encoun- 

 tered. In the tropics we have the palms, which give so striking a charac- 

 teristic to the forests, the broad-leaved bananas, and the great climbing 

 plants, which throw themselves from stem to stem like the rigging of a 

 ship. Next follows a zone described as that of evergreen woods, in which 

 the orange and the citron come to perfection. Beyond this, another of 

 deciduous trees the oak, the chestnut, and the fruit-trees with which, 

 in this climate, we are so well acquainted, and here the great climbers of 

 the tropics are replaced by the hop and the ivy. Still farther advanc- 

 ing, we pass through a belt of conifers firs, larches, pines, and other 

 needle-leaved trees, and these, leading through a range of birches, which 

 become more and more stunted, introduce us to a region of mosses and 

 saxifrages, but which at length has no tree nor shrub ; and finally, as 

 the perpetual polar ices are reached, the red snow-alga is the last trace 

 of vegetable organization. 



A similar series of facts had been observed by Tournefort in an ascent 



