THE CLIMATAL DISTRIBUTION OF BRITISH PLANTS. 51 
of the competition—the struggle for existence between the 
different forms. The plants which grow in the higher, and, 
consequently colder stations, do not grow there because 
they particularly like cold. The true reason is that while 
they can live there, others cannot, and thus they have the 
ground to themselves, while, in the lowlands, they would 
be crowded out. 
Besides height above sea-level, there is another general 
relation of situation which causes a difference of temperature, 
and thus affects the flora in precisely the same way. This 
is Latitude. In consequence of this, a tour from the South 
of England, northward, would show a very similar suc- 
cession of plants to that seen while ascending a mountain. 
The change however, as we proceed northwards, is very 
gradual; it takes several hundred miles in a north and 
south direction to show the same effect on temperature and 
flora as will be seen in a climb of a few hundred feet up a 
mountain side. Thus, the difference in botanical climate 
between the South Coast of England and the Shetland 
Isles, is no more than that between the Trent Valley and 
the summit of the Peak of Derbyshire. 
For the systematic working out of the climatal distri- 
bution of our British flora we are indebted to the late 
Hewett Cottrell Watson. This eminent Botanist in his 
great work Cybele Britannica divided the surface of the 
country into two great Regions by tracing a line along the 
upper limit of growth of cultivated crops; the region which 
lay below this line, that in which corn or other cultivated 
crops were, or might be grown, he called the Agrarian 
Region, or the region of cultivated crops; that which lay 
above this line was the country in which the severity of the 
climate was such that cultivation could not be profitably 
carried on, this he called the Arctic Region, or the region of 
alpine plants. 
Each of these regions he then divided into three Zones, 
