THE GEOGRAPHICAL DISTRIBUTION OF PLANTS. 105 



and which has tlie greatest extremes of temperature of auy pU\ce 

 iu which observations of this kiml have been made. 



(3) The height of land above the sea-level alwaj's causes, ceteris 

 paribus, a direct variation in temperature, this falling as the height 

 increases, and of course rising as the height diminishes. From 

 this cause it happens that we find upon high mountains, a series of 

 stages in the vegetation, the characters of which indicate succes- 

 sively colder and colder zones. And usually, as all botanists know, 

 the vegetation of these successive zones of altitude corresponds 

 closely to the vegetation of those places farther north, which at or 

 near the sea level have the same mean of temperature ; in other 

 words we have zones of altitude corresponding very exactly to zones 

 of latitude, and wherever the late geological history of a region has 

 permitted of it, the vegetation of these two kinds of zones corres- 

 ponds not only in general characteristics, but is composed of 

 identically the same genera and species. Hence it is that we find 

 arctic plants upon the Alps and Rockies, sub-alpiue plants upon 

 the White mountains, and Antarctic plants upon the Andes — a 

 subject soon to be referred to again. 



The influence of the first of these conditions, must be prodigiously 

 altered by the second and third, and the effect will be to make the 

 zones of temperature extremely irregular. Humboldt invented a 

 verj' simple method of graphicall}" representing the boundary lines 

 of these zones ; that of drawing upou a map isotherms or isother- 

 mal lines ; and any map upon which these are drawn will show very 

 clearly the effect of the second and third of the conditions mentioned 

 upon the first. And it is important to notice that all of these con- 

 ditions operate as well upon a small as upon a large scale, though 

 in diminished degree, and therefore distinguishable with more diffi- 

 culty. 



It is necessary to note, in connection with this subject of tem- 

 perature, that the whole matter is complicated somewhat by the 

 fact that the distribution of plants depends quite as much upou the 

 average temperature of the growing and reproducing mouths as 

 upon the average annual temperature — perhaps, indeed, a good deal 

 more. This serves to explain those cases iu which grains will 

 thrive iu regions very far north of where, from their mean annual 

 temperature, we should expect ; this seems to occur wherever the 

 actual average temperature of the actual growing time is high, even 

 though that of the remainder of the year is disproportionately low, as 



