HOW CLIMATE AFFECTS VEGETATION. 165 



to say something in regard to the distribution of forests on the earth's sur- 

 face, and the climatic conditions which accompany and may be assumed to 

 have influenced this distribution. But a limited amount of investigation is 

 required in order to have it made clearly apparent to the candid mind that 

 the character of the flora of any region is most powerfully influenced by 

 variations of the climate, and especially by changes in the temperature and 

 the amount of moisture. Summed up in a few words, it may be stated that 

 extreme cold and extreme dryness are unfavorable to the development of 

 vegetation. In the case of temperature we see this almost equally Avell 

 illustrated, whether we journey toward the Polar regions, or rise on the 

 sides of lofty mountains, the decrease of temperature manifesting itself, in 

 a most marked degree, by corresponding changes in the vegetation. The 

 forest trees which are recognized as typical of warm climates disappear; 

 those characteristic of colder regions make their appearance. These, in 

 their turn, become more sparsely distributed and dwarfed in size, and finally 

 give out altogether ; some grasses and flowering plants maintain their hold 

 up to still higher and colder latitudes ; and finally all these disappear, and 

 only the lichens remain, of which no land, however far north it may lie, has 

 ever been found entirely destitute. Of a similar character is the decline of 

 vegetation as we ascend the slopes of high moimtains. Trees first disap- 

 pear ; higher up, grasses and flowering plants do the same ; while the lichens 

 maintain their hold to the last, and often imtil the line of eternal snow is 

 reached. That these eflfects are mainly due to temperature changes can 

 hardly be doubted. The disappearance of the trees is coincident with a 

 diminution of tlie temperature, and is not accompanied by a corresponding 

 falling off in the amount of precipitation. On the contrary, the giving out 

 of the arboreal vegetation may and does take place where moisture is abun- 

 dant, as on the slopes of high mountains between the forest line and the 

 snow line. The same condition reveals itself most clearly when we consider 

 carefully the position of the timber-line along the northern edges of the 

 great land-masses of the northern continent. 



But, on the other hand, it is not possible to deny that the presence or 

 absence of moisture has much to do with the character of the vegetation ; 

 and no one can doubt that the distribution of forests over the earth's surface 

 is largely dependent on the position of the areas of greater or less precipi- 

 tation. A very large rain-fall may coexist with an abundant forest growth ; 

 and so, as it appears, an abundant arboreal growth may be found in regions 



