CH. V] HINDER THE DISPERSAL OF SPECIES 45 



conditions. The plants in the mountain garden in Ceylon are 

 very different from those in the gardens in the "low" country, 

 not from any special wish to keep the collections distinct, but 

 from the great permanent difference of 20° (F.) in the mean 

 temperature, though the highest and lowest of both stations may 

 easily be reached in the same day at the same place in Europe 

 or North America. 



Every function in every plant has a temperature (the mini- 

 mum) below which it will not go on, a temperature (optimum) 

 at which it will best go on, and a third (maximum) above which 

 it ceases. As these differ for every species, one kind of climate 

 will suit one, and not another, though there is no doubt that 

 species may become acclimatised (cf. Chapter iv). If the ex- 

 tremes of temperature come at a season when the functions 

 concerned are not being performed, they may be easily with- 

 stood, as for instance the great cold of winter in North Siberia, 

 which does not kill the conifers there. Extreme cold, when un- 

 seasonable, does at times kill out species, but the loss is usually 

 recoverable, especially as it is only necessary for the plant to 

 regain a foothold in societies of plants of Avhich it has already 

 been a member. 



Light, again, changes too gradually from place to place for it 

 to be supposed that it has any appreciable effect in opposing 

 a barrier to any species. Species from one part of the equatorial 

 tropics do just as well in another part Avith much less intense 

 light, or vice versa. It is in general only in descending into deep 

 water that there is any great change in light over a large area, 

 and even there some plants are found below the limits of darkness. 



Wiiid is chiefly of importance in an indirect manner, according 

 to whether it is wet or dry, and according to its direction in 

 reference to that of the mountains, but if very strong, it may 

 alter or prevent the growth of some species. On the west coast 

 of Britain one may often see trees blown into a one-sided type 

 of growth, and a little more wind would prevent their growth 

 altogether. A cyclone may uproot so many trees that it may 

 render passage through a country possible for herbs which can 

 quickly seize upon the vacant spots before the growth of the 

 forest once more suppresses them. 



Though climatic differences are thus of such enormous im- 

 portance one must be careful not to say of any species that it 

 has certainly reached its climatic limit, when one has regard to 

 the very slow and gradual acclimatisation that is practised by 



