585 



further south it is so much at home, that it yields one of the necessaries 

 of life to the entire population. Oranges will ripen on the other side 

 of the Alps, but not on this. For those cereal grains, those corn- 

 plants furnishing the principal portion of the food of man, we find 

 distinct lines of demarcation extending across Europe, beyond which, 

 northward, each kind ceases to be capable of ripening its seed. Of 

 trees we know that certain kinds will flourish and form fruits at points 

 far north, where others are arrested by the cold ; the firs, for instance, 

 exclusively constitute the most northern woods of Scandinavia, while 

 the dwarf palm, a representative of tropical climates, maintains its 

 fooling even so far into the temperate region as Italy and the south- 

 ern confines of France. 



" Again, as indeed must be perceptible to every one who has visited 

 mountainous countries, vegetation alters in its characters at different 

 elevations, and it has been shown that these variations correspond to 

 those which are observed on the level plains in proceeding from the 

 south towards the north ; the increased severity of the climate of the 

 higher localities acting exactly in the same way as the colder climate 

 of the regions lying further from the equator. 



" Such facts as these, obvious as they appear to be, remained un- 

 connected and unaccounted for until recent times, or differences of 

 heat and cold were supposed to be sufiicient to explain them. But 

 when a more searching inquiry arose, and when the vague ideas 

 respecting the influence of heat came to be systematically investigated, 

 it was found that there were other facts and that other causes were at 

 work, the existence of which had not previously been suspected. In 

 the first place it was seen, that mere degree of latitude will not indi- 

 cate the temperature of a climate ; that the temperature, the average 

 heat and cold, do not alone constitute the climate properly so called, 

 but that humidity, exposure to pievailing winds and many other influ- 

 ences conjoin to produce the atmospheric conditions powerfully affect- 

 ing vegetation. The chemical and physical conditions of the soil 

 were found to require investigation, in order to the explanation of 

 facts otherwise anomalous ; and finally it has been discovered that 

 the particular constitutions of the individual species of plants must 

 be studied, if we would rightly understand the causes which give the 

 peculiar characters to the vegetation of different lands. 



" And after all these points have been considered, there is still a 

 residuum of phenomena which they totally fail to account for ; we 

 are in possession of another series of facts which require to be 

 VOL IV, 4 F 



