24 Proceedings of the Royal Physical Society, 
crop-plants grown closely together, we find that orchards have certain insect 
and other pests closely associated with them without the natural restrictions 
found in virgin associations, and these have to be specially guarded against 
wherever orchards are planted. 
On the other hand, the spread of a southern fauna into formerly isolated 
moorland districts has sometimes led to disaster in the extensive pure 
associations of our native birch woods. Thus planted associations are liable 
to destruction from the unevolved and unbalanced condition of their vegeta- 
tion and insect or other fauna, but naturally evolved associations only 
suffer when their natural balance is disturbed. 
The grasslands, marshes and fens of the low-lying alluvial stretches have 
had much of their original plant associations eradicated by drainage or by 
the plough. Even the coastal sand dunes, so long almost disregarded by the 
economist, are now being remorselessly civilised by the golfer and the 
mowing-machine, so that many close adaptations and associations of plant 
and animal life to their surroundings will be rapidly replaced by patches of 
weeds and artificially formed grassland. 
Wide stretches of heath and moor, owing to greater difficulties in their 
profitable reclamation, have stubbornly repelled the attempts of the 
agriculturist to annex their territory, and certain grassy uplands of the 
chalk downs have passed into his keeping without suffering much change. 
The wilder moorland still forms a sanctuary for the grouse and mountain 
hare with their attendant foxes, peregrines and eagles, but much of it has 
been turned into deer-forest where unnatural numbers of red deer are kept in 
a semi-domesticated condition, freed from the enemies that formerly exercised 
a more selective control upon their destiny. 
By ruthless forest exploitation, by burning the heather, artificial drainage, 
overstocking with deer, by the introduction of sheep and rabbits, and by the 
destruction of the natural predatory birds and mammals, much alteration has 
been effected even in remote parts of our moorland, and many associations 
have succumbed and others are driven to the verge of extinction. 
2. THE DIFFICULTIES OF THE PROBLEM OF DISTRIBUTION, AND THE ARTI- 
FICIALITY OF THE PRESENT METHODS OF RECORDING DISTRIBUTION, ARE 
CHIEFLY RESULTS OF THIS REPLACEMENT. 
The problem before the ecologist in this country is thus rendered very 
complex and difficult, since his business lies in attempting to solve the 
natural relations of living things to their habitats (1). Had civilised man 
been long enough in possession to adapt the entire surface to the ideals of 
the radical economist, and to completely obliterate the state of things that 
