30 ACCLIMATISATION [pt. 



different conditions that may be experienced in a transit of a 

 few score of yards or less, but small journeys like this, added 

 up over many centuries, ultimately result in enormous differences 

 of conditions, as when one finds Hydrocotijle asiatica growing in 

 the low country of Ceylon, with a steady mean temperature of 

 80°, and in Stewart Island, New Zealand, with Avinter snow and 

 frost. 



Acclimatisation may also take place in nature without the 

 plant changing its position, by the secular changes of climate 

 which are usually going on. New Zealand had probably at one 

 time a more or less tropical chmate, and now has a temperate 

 one, yet the tropical species are still to be found there, and quite 

 probably may have originally arrived when the climate was 

 warmer, and then become gradually acclimatised, themselves 

 and their descendants, to climates steadily becoming colder. 

 The rise of a mountain chain may gradually acclimatise plants 

 to a colder climate, by carrying them upwards. 



This gradual acclimatisation that is carried on by nature has 

 often been so successful, as illustrated by Hydrocotijle asiatica 

 above, and by scores of other "tropical" species which are found 

 far south in cold but still damp climates (to the northwards the 

 change to dry is more sudden) that it makes it very difficult to 

 say when a species has really reached its climatic limit, beyond 

 which no amount of acclimatisation would be of any use. People 

 are apt to say that laboratory experiments show that such or 

 such a temperature is the lowest that a plant will stand, for- 

 getting nature's very gradual acclimatisation. Hydrocotyle from 

 Stewart Island would almost certainly gi^e reactions in the 

 laboratory different from those of the same plant from the 

 plains of Ceylon. 



It is possible, again, that in nature's acclimatisation by 

 gradual change of climate, plants may become slower in the 

 performance of their functions, or in groAvth, so that the genera- 

 tions may be farther apart. 



Yet another factor that has probably an important influence 

 is the increasing number of species upon any given piece of 

 country. As the species increase in number, they probably begin 

 to form more or less complete or "closed" associations of plants, 

 into which intrusion of a newcomer becomes increasingly diffi- 

 cult, so that probably both rate of travel and acclimatisation 

 are rendered slower and more troublesome. 



