INTROPUCTORY EXPLANATIONS. 31 



evitably sever and divide that which is nearly alike ; the 

 vegetation being more similar on the contraiy sides of any 

 one dividing line, than it is on the two sides of the broad 

 zone between any two lines. This disadvantage attends all 

 our arrangements and groupings of nature's realities. We 

 see it in the gi'ouping of plants into orders and genera ; we 

 see it in grouping them together according to their pro- 

 vinces and counties ; we see it in an'anging them under 

 climatic zones. But this disadvantage is still far out- 

 balanced by the practical convenience of such groupings 

 and arrangements, in the acquisition and commmiication of 

 knowledge. The human mind cannot grasp all nature at 

 once, either as one gi'eat whole, or in its innmnerable de- 

 tails. The gi-eat imit must be divided into parts ; the de- 

 tails must be combined into groups ; without our losing 

 sight of the tnith, that all these parts or groups do gradu- 

 ally unite and merge into each other. 



It wlU be recollected also that chmatic or ascending 

 zones of plants are designed to indicate the relative distribu- 

 tions of species under the joint influence of altitude and 

 latitude, with other conditions of a more local chai^acter, 

 such as proximity to seas or mountains, the state of ex- 

 posine or shelter, &c. Connexions may be traced between 

 the distribution of plants, and each of these conditions 

 singly ; but everywhere the influence of any one condition 

 is more or less distm'bed and modified by that of others. 

 Then- effect upon the flora or general vegetation is mostly 

 indirect or remote ; that is, the flora varies fldth the climate, 

 and the climate varies with altitude, latitude, and other 

 conditions of place and sm-face. On a single isolated 

 moimtain the ascending zones of vegetation are very strongly 

 mai'ked, in accordance with latitude ; some species disap- 

 pearing, other species appearing, one above another, as we 



