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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. 
In 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- 
