GEOGEAPHY OF PLANTS. 473 



of Mount Ararat. He found that the distribution of the veg- y ertical dis _ 

 etation from the base to the top of the mountain bore a gen- tribution of 

 eral resemblance to the distribution from the base toward the p a 

 Arctic regions. These facts by subsequent observers were generalized, 

 it having been established that there exists an analogy between horizon- 

 tal distribution on the surface of the globe and vertical distribution at 

 different altitudes above the level of the sea. Even in the tropics, if a 

 mountain be sufficiently high, a very short ascent suffices to carry us 

 from the characteristic endogenous growths at its foot, in succession, 

 through a zone of evergreens into one of deciduous 1 trees, and this, again, 

 into one of conifers, the vegetation declining through mosses and lichens 

 as we reach the region of perpetual snow. 



In these two cases of horizontal and vertical distribution respectively, 

 which thus present such a striking botanical resemblance, Distribution 

 there is likewise so clear a meteorological analogy that it is ofheatdeterm- 



, . , , ines the distri- 



impossible to avoid coming to the conclusion that the dis- button of 

 tribution of plants depends on the distribution of heat. The P lants - 

 same climate variation encountered on a surface journey directed from 

 the equator toward the poles is again encountered as we leave the foot 

 of a tropical mountain and go toward its summit ; for it is a well-ascer- 

 tained fact that the temperature of the atmosphere declines as we rise to 

 greater altitudes, and that, no matter how high the summer heat may be, 

 we may, by a vertical -ascent at any locality, come to a region where 

 the temperature is never above 32 Fahr., and where ice and snow, there- 

 fore, never melt. If, in any locality, the mountain ranges are of sufficient 

 height to gain that region, their tops will be covered with perpetual 

 snow. The vertical ascent thus to be made is less as the latitude is 

 greater. At the equator it is 15,200 feet, and at the eightieth degree it 

 is within 450 feet of the ground. Beyond this, the surface itself is per- 

 petually frozen. 



The mean temperature of a place determines its vegetable growth, and 

 hence there will ever be a resemblance between the vegetation of places 

 of the same mean temperature, though they may be geographically very 

 wide apart. But this, though a resemblance, is very far from being an 

 identity. We can not always designate by name the particular plants of 

 a high latitude which should be found at a corresponding elevation in the 

 mountains of the tropics. There may be the general resemblance of 

 which we have been speaking, and yet the genera and species of plants 

 in the two places may be quite distinct. But this fact, far from affecting 

 the truth to which we have arrived of the control of a physical agent 

 such as heat over the distribution of plants, leads us to ex- Influence of 

 tend it, and teaches us that, though we might expect, in other physical 

 places far apart, identically the same vegetable growths if conditions - 



