XI. CLIMATAL ZONES. 483 



at some length in the introductory portion of volume 

 first, pages 19 to 43. Under such an arrangement the 

 plants are placed in series, and divided into successive 

 groups, in accordance with their geographical position 

 relatively to each other. Such relative position appears 

 to be determined by the climates to which they are re- 

 spectively best adapted, and by the geographical peculi- 

 arities of the countries in which they are found. But the 

 actual conditions, climatal and geographical, are combined 

 in proportions so countlessly varied that they cannot be 

 exactly ascertained for any single species separately, and 

 still less can they be determined precisely for groups. 



The influence of latitude and longitude, elevation, 

 insularity, montane proximity, and other conditions, has 

 been alluded to more or less fully on former pages ; as 

 also the partial extent to which altitude and latitude 

 actually compensate for each other, in their influence 

 on the floras of different countries. Climatal zones 

 are designed to express the complex result from those 

 conditions in their ever-varying proportions. But such 

 zones are more conveniently, because more simply, traced 

 out by reference to the relative limits and positions of the 

 plants themselves, with only subordinate reference to 

 their more special connexions with climate and geo- 

 graphy. 



In attempting to make out such a zonal arrangement 

 for the flora of a whole country, which includes a con- 

 siderable range of latitude and much inequality of surface, 

 we proceed on the assumption that latitude and altitude 

 do in the main compensate one for the other, — that on 

 attaining a higher level in a more southerly district, we 

 should mostly find there the same species that we should 

 also find by a journey of sufficient latitudinal distance 

 northwards, — that the higher the elevation of tlie more 



