22 Origin of the British Flora. 
was only comparatively small area suited to their 
needs. 
Before studying more minutely the means of dispersal 
available, it may be well to ask, in this connexion, what 
are the requirements that are usually essential to the life of 
the species. In the first place, it is necessary that the 
seed should be sown beyond the limit of the patch of soil 
exhausted by the parent plant. For this a very slight 
mobility is requisite. Secondly, in the case of British 
plants, some method is ordinarily needed by which 
they are enabled to cross barriers, such as rivers or 
straits, or tracts of desert in which the plant cannot 
flourish. 
I use the term ‘desert’ as implying areas unfavourable 
to any particular species. A desert from the human 
standpoint is a sandy waste without water, which is 
unsuitable for the plants and animals useful to man. 
Such an area may be gay’ with flowers, and is no desert to 
the Gorse or Horned Poppy—the desert to them is the 
luxuriant meadow or forest, which they cannot overpass 
unless their seeds are carried by some rapid messenger. 
To a water-plant the dry land is a desert ; to a mountain 
plant the lowlands are desert ; to the lowland plants the 
mountain is a desert ; and to go further, to certain plants 
everything but limestone rock is a desert. Consequently 
the British Isles consist not only of an Archipelago with 
numerous islands, but from the points of view of different 
plants the area forms quite different Archipelagos, of low- 
lands with scattered mountain tops, of non-calcareous 
country with isolated limestone, or of dry land with scat- 
tered lakes. 
In gregarious plants, such as heaths and rushes, the 
necessity for scattering the seeds beyond the shadow of, 
and beyond the soil exhausted by the parent species, may 
