Ecology. 31 
restrictions, or by a newly acquired licence. It is therefore probable that 
each type of habitat originally held a well-marked animal and plant 
association far more distinct than we find now. 
If a patch of ground is cleared in the midst of natural vegetation of a 
stable type, it is rapidly invaded by plants from two quarters—(1) plants 
from migratory associations in the vicinity, and (2) plants from the surround- 
ing stable associations. 
Provided no further interference takes place, the plants of the stable 
associations may gradually oust all other competitors, and the stable formation 
is reconstituted. 
If the interference is repeated, a number of plants from the migratory 
associations continue to contest the ground, and only a few of those from the 
stable associations obtain any footing. If the interference is periodic and 
of an evolutionary nature, such as only occurs where natural migratory 
factors are in operation, migratory associations take possession of the ground 
and reach that degree of stabilisation allowed by the migratory factors. 
If, instead of merely a patch of ground being cleared of its original vegeta- 
tion, a wide district gradually undergoes a transformation of this kind, the 
immigration of plants from the original stable associations is checked and 
eventually eliminated, and the ground. which formerly supported stable 
associations, if left to itself, can only become clothed by types of vegetation 
constructed from such plants as were the least easily exterminated, or the 
most rapid in migration. ‘These are essentially those of migratory formations, 
- since their habitats are generally governed, even in the most fertile districts, 
by geological factors which have hindered their complete reclamation by man. 
In accordance with this, we find limited artificial clearings in our moor- 
land districts are soon recaptured by the moorland associations, while those 
in widely cultivated districts are often, for long periods, occupied by a motley 
assemblage of weeds, incapable of reconstructing any associations comparable 
with those of the original vegetation. 
But certain types of migratory associations depend on the influence of 
grazing animals for their maturation and persistence to an even greater 
extent than on the geological factors, which were often an initial cause in 
their formation ; and chief amongst these are types of grassland which largely 
owe their degree of stability of association to the constant presence of such 
animals. From this, no doubt, has arisen the tendency for the plant 
associations, which gather on the waste or fallow ground of our cultivated 
districts, to stabilise as some form of grassland wherever the physiography 
and soil allow. The evolution of some degree of stability in grassland is 
probably as ancient as the evolution of grazing animals, and grasslands must 
