Introduction. 



The flora of a particular country or district may in the main 

 be divided more or less distinctly into three groups of plants, viz. : — 

 (1) Those plants which reach their maximum in the area in question^ 

 i. e. occur as frequently as circumstances permit, or are at any rate 

 widely distributed there ; (2) those which are the advanceguards of 

 the intruding vegetation ; and lastly] (3) those which are the survivors 

 of an earlier vegetation that is now becoming extinct. Within the 

 first group, which doubtless includes the greater part of the species 

 occurring in the district, we may distinguish between those which 

 have recently migrated thither, and those which are descendants 

 of an old vegetation of the district that have adapted themselves to 

 the climate, continually although slowly changing. Generally, when 

 a plant migrates into a particular district we may assume that the 

 climate of the latter suits it. But this does not imply that all the 

 different parts of its structure are in correlation with the climate; 

 this is not necessary, and such features may often be sought for in 

 vain. The plant may have a protoplasm which does not require 

 special external contrivances, as for example organs that tend to check 

 transpiration. On the other hand it may in most cases be said of 

 a vegetation that is dying out, that it is becoming extinct owing to 

 the climate offering it too much opposition. In the latter case a 

 protoplasm will be found which is less capable of resistance, as 

 also external structures which are in every way more specialized, 

 and which in combination with a certain lack of power in the 

 protoplasm to adapt itself to the new conditions, are unable to 

 maintain the life of the plant in the region in question. 



Starting now from the view given above, — that a plant 

 which migrates into a particular district need not from the first be 

 provided with special contrivances suited to the climate of the 

 country, but, on getting foothold in the district, will change more 



