134 PLANT LIFE 
which saturates the soil of peaty moorlands, 
is not available for the vast majority of 
plants, and they are consequently precluded 
from occupying regions where such conditions 
prevail. Those plants which do tolerate or 
even demand them, often take in relatively 
small quantities of the water, and they have 
in consequence to limit the amount lost as 
vapour in transpiration. In connection with 
this limitation a variety of subsidiary modifi- 
cations of habit may become manifest. Slow- 
ness of growth, succulence, or the opposite 
character of spininess, are common features; 
whilst an evergreen habit with leathery leaves 
is of fairly frequent occurrence amongst the 
perennial plants of such localities. Indeed, 
experience shows that any circumstance tend- 
ing to reduce the amount of available water, 
whether due to physical or physiological 
conditions, will stamp its impress on the 
vegetation. 
The onset of a period of drought will 
speedily result in the extinction of entire 
species within the affected area, and their 
places will rapidly be taken by others which 
are already adapted to these new conditions. 
Which of the many possible forms of adapted- 
ness to drought a particular colonist may 
possess, depends of course on its own inherent 
and hereditary properties. The part played 
by the environment in the matter is merely 
to rule out all those plants which are not 
previously fitted in one way or another to 
