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or less in response to the existing climate and may adapt itself to, 

 or be more directly influenced by, the new conditions — we must 

 arrive at the conclusion that to ascertain the influence of the climatic 

 factors of a particular district on its plant-world we must investigate 

 the vegetation which is the oldest in the area, and which thrives 

 well there at the present time. But also in this case, the lesser 

 or greater power of the protoplasm to utilise external contrivances 

 must be taken into consideration. If a plant needs only slight pro- 

 tection against, for example the danger of excessive transpiration, 

 then of course it cannot give us any information regarding the 

 influence of climate on the structure of plants. From the above 

 it will be seen why it is desirable that plant-geographical studies 

 should go hand-in-hand with studies in the lesser or greater adapta- 

 tion of the plant-world to climate. Forms with a limited distribution 

 are of course the best objects of study with regard to ascertaining 

 the special demands of a climate over a long period, but species 

 with a wider distribution may also be useful in regard to this point. 

 Here it must be borne in mind that the fact of the occurrence of 

 a species in different climates need not exclude it from being used 

 in the above-mentioned studies. As we know, species often change 

 their locality according to the climate, so that the different external 

 factors in the different localities amount to the same sum total. 



hi regard to all plant-migrations, the power plants possess 

 mutually to supplant each other of course plays a prominent part; 

 therefore it may be asked, whether the components of the vegetation 

 of any area which are becoming extinct may not in themselves 

 reflect the existing climate of the area in question. This cannot 

 be denied. It is however my opinion, that but rarely are plants 

 entirely expelled from a particular area by other plants, the reason 

 for ultimate extinction usually lying in the fact that the relationship 

 between the life-functions or the external organs and the external 

 factors is faulty. With regard to the influence of climate on the 

 plant-world it must be pointed out, that it is difficult to arrive at 

 any isolated result; the influence of a climate cannot be expressed 

 in a single sentence, the climate not being constant itself during 

 the year, and the plant-world not consisting of homogeneous ma- 

 terial. Hence , the different plant-types must always be kept 

 distinct and treated separately. To keep the influence of the individual 

 localities distinct from the climatic factors proper is also doubtless 

 very difficult. In beginning to enter upon studies of the kind men- 



