IV. EXPLANATIONS. Ji'/l 



latitude. Each condition, latitude and altitude, was there regarded by 

 itself, without consideration of the other. In this manner Vlex exiropmis 

 and Azalea procumbens got placed under the same divisions of latitude 

 aud of elevation, the northern and the ascending, although in fact they 

 do not belong to the same cliinatal flora. The Vhx ascends above 

 700 yards in Wales, and the Azalea descends below the same 

 height in North Britain ; thus actually growing on the same level 

 of altitude, if the scope of view takes in the whole of Britain ; although 

 not anywhere meeting on the same level of height under the same 

 degree of latitude. In the third series of orders, now under explana- 

 tion, this and various other similar anomalies disappear ; through com- 

 bining the plants of higher levels in more southern latitudes, with 

 those of lower levels in more northern latitudes. In so far, the 

 two regions or six zones supply more suitable data for tracing the 

 influence of climate, in varying the proportions between the groups of 

 systematic botany, than would be supplied by zones of latitude or alti- 

 tude, if regarded by themselves only, and not in modifying connexion 

 with each other. Zones of latitude might sufiice equally well or better, 

 if they could be traced across a country of nearly the same unvaried 

 elevation. Or, zones of altitude might well suffice for climate also, if 

 traced on a single mountain or group of mountains, whose base covered 

 only a small horizontal area of latitude and longitude. 



A cursory glance over the figures in this climatal series, will show 

 that wide floral difl'erences can be traced between the two regions and the 

 six zones. From the agrarian to the arctic region, from the lower to the 

 upper zones, there is a general decrease in the flora, and of course a 

 corresponding average decrease in the orders. This decrease is attri- 

 butable partly to the decreasing area, as will presently become apparent, 

 and partly to the deteriorating climate. But it will be observed that the 

 ordinal decrease is extremely unequal and disproportionate, when the 

 orders are comp red one with another. For example, Leguminiferce 

 exceed Crucifera by one-sixth in the general flora of Britain : and 

 nearly the same disproportion is observable in the floras severally of the 

 agrarian region or lower agrarian zone. In the upper agrarian zone 

 the numbers become equalised ; while above the lower arctic zone the 

 Crucifer<B predominate even in absolute numbers ; thus reversing the 

 relative proportions of the lowest zone. 



