ADAPTATION TO ENVIRONMENT 35 
characteristic members of the woodland association of 
one or more formations. But with them we shall find 
other species, such as the Wild Strawberry (Fragaria 
vesca), which are equally at home on dry sunny 
banks or even on sand dunes. 
If we ask why the plants group themselves into the 
associations which we may study any day in the 
country, in many cases the answer is not obvious. 
It is clear that while many species accommodate them- 
selves easily to different soils or different degrees of 
light or of moisture, others have small powers of 
accommodation, and are in consequence restricted in 
their range. By long usage many plants have 
acquired special characters enabling them to live 
under special conditions—some examples will be 
discussed a little later—and in some such cases 
it is easy to correlate the peculiar characters of the 
plant with those of the habitat. But in many other 
cases the relation is not obvious. For instance, we 
cannot tell, by examining a plant, whether it is partial 
to a limy or to a non-limy soil; yet many plants are 
poisoned by lime, while others, though generally 
capable of growing in a soil devoid of lime (if 
planted in a garden), are nevertheless absent from 
the non-calcareous areas adjoining their limestone 
habitat; in other words, they can hold their own on 
limestone, but are unable to do so elsewhere. The 
two ferns already mentioned (Polypodium Roberti- 
anum and Lastrea rigida) are cases of the latter 
kind; while some of the most familiar of our hillside 
plants, such as Foxglove (Digitalis purpurea) and 
Broom (Sarothamnus scoparius), are instances of the 
former. 
