CONCLUSION 305 



the spruce forests from east to west through north- 

 western Europe is an established fact. Gradual changes 

 of climate are almost certainly responsible for the 

 retreat of the Canary temperate rain-forests to secluded 

 moist gorges ; for the transformation of the vegetation 

 of Cyprus and Cyrenai'ca, the latter once a granary of 

 Rome: again, travellers through the deserts of Takla- 

 makan report having passed through endless miles of 

 dead scrub of saxaul and other desert shrubs. 



That the length of time during which the present 

 physical conditions have prevailed, and therefore during 

 which the process of adjustment has been going on, has 

 been unequal in various corresponding regions, is highly 

 probable and, in some instances, is established beyond 

 doubt. The result naturally is that the aim is not at- 

 tained with the same completeness everywhere and that 

 a more or less large proportion of plant-forms belonging 

 to previous periods in the history of these regions has 

 been able to survive. In other words, when dealing 

 either with single growth forms or combinations of them 

 into landscapes, the problem involves the fourfold 

 question : 



(1) of origin, starting-point, or previous history; 



(2) of process of adjustment ; 



(3) of time during which this process has been at 

 work ; 



(4) of the gradual evolution of physical conditions ; or 

 again, of the point which the process of adjustment 

 may have reached. 



The task of finding out homologies is thus rendered 

 somewhat complex, and the degree of correspondence is 

 not equal in all cases. 



Last, not least, man has been instrumental in intro- 

 ducing profound changes in the character and economy 



