138 PLANTS AND MAN 
colonization of the Romans may well have been 
accountable for the introduction into Britain of many 
of the farm plants still grown there. The wars of the 
next fifteen hundred years on the one hand, and the 
spread of agriculture on the other, caused the steady 
destruction of the forests, till at length England and 
Central Europe began to assume their present appear- 
ance. The draining of marshes and fens, the enclosing 
of land, went on steadily, and to a slight extent is 
going on still; within recent years, the European War 
has resulted in the disappearance of many of the 
remaining woods, and in the breaking up of fresh 
land. 
From the point of view of the botanist, agriculture 
consists of the destruction of the plant associations 
which for some thousands of years have occupied the 
ground, and their replacement by other plants which 
are useful to man. The natural plant associations 
being the result of the survival of the fittest through 
a long period of time, while the farmer’s crops 
represent plants which do not grow naturally on the 
ground, nor often indeed in the country (while they 
are frequently artificial forms unable to reproduce 
themselves), it follows that the latter cannot compete 
with the former, and can be maintained only by the 
most careful protection. The native plants are always 
striving to reoccupy their legitimate territory, and the 
farmer is incessantly engaged in trying to keep them 
out. Agriculture, indeed, has been defined as “a 
controversy with weeds.” Incidentally, the suppres- 
sion of the natural flora allows many weaker plants 
an opportunity of which they are not slow to take 
advantage, These may be natives, but are often 
