SPECIFIC WEIGHT. 55 



of its production both towards cooler or warmer regions. 

 It is indifferent whether the annual zones consequently 

 increase or decrease in breadth, or whether the wood is 

 broadleaved or coniferous. Within the natural habitat of 

 any species of tree the centre of its habitat produces the 

 heaviest and hardest wood. 



Every species of tree lives in a certain climatic region, 

 although the habitat of the tree may show great irregulari- 

 ties owing to marine currents or topographical features. Such 

 irregularities in the habitat of a tree, insular expansions on 

 the one hand or insular exclusions on the other, might induce 

 one to suppose that not the climate, but the soil, is decisive 

 for the natural extension of the species. 



[There is a close relation between a tree's demands upon 

 temperature and upon soil. Given the proper temperature, it 

 will grow where the soil is unfriendly; and given the most 

 congenial soil, it will grow where the temperature is not ideal. 

 The colder and wetter the soil, the better will a tree grow with 

 a relatively high temperature ; the drier and warmer the soil, 

 the better .it will grow with a relatively low temperature. 

 Thus on a northern slope the forester will often find it safe to 

 plant trees which would not thrive on the southern side of the 

 same mountain, because northern slopes are cooler and moister 

 than southern ones, and this difference may suffice to effect a 

 slight reduction in the average temperature of the region. 

 There is a wide variation among trees as to the range of tem- 

 perature which they endure. But it should not be inferred 

 that only geographical lines can be drawn for the distribution 

 of any species. The right temperature conditions may be 

 found outside the geographical habitat at higher or lower 

 altitudes. A southern species, w r hose home is in the moun- 

 tains, may find a second home in the northern latitudes of a 

 level country, and a northern lowland species may thrive on 

 mountains in the south. Tr.] 



Mayr's observations here (cf. p. 60) do not include the soil 

 as one factor in this natural extension, but regard the range of 

 temperature and the distribution and amount of annual rainfall 

 as its most important factors. Hence arises the important fact 

 for the cultivation of trees that there may be climatic regions 



