Ecology. 27 
made ground. But a proper knowledge of physiography and 
natural history enables the fieldsman to detect much finer degrees 
of interference such as may be passed over by one who lacks this 
knowledge, and so we have as a second group— 
(2) Altered by drainage, burning, stocking with animals, planting and 
cutting timber, mowing grass, or from the action of smoke cloud. 
The results of such interference may merely cause a preponder- 
ance of certain species or natural types of association, or may 
lead to new or secondary associations having an approach to the 
artificial. 
(3) Natural, which may be further subdivided into (1) Stable, those that 
are comparatively stable and carry such vegetation as moor, 
wood, heath and certain types of grass in this country. (2) 
Migratory, where geological or animal factors locally dominate 
the station and induce such vegetation as marsh, fen, alluvial 
grassland, bushswamp, etc, (3) (4). 
The naturalist naturally searches places where he obtains the greatest 
number of species in the least area, and these are almost always occupied by 
groups of migratory formations. Stable formations yield comparatively few 
species after the first mile or so has been exhausted. This not only applies 
to the plants but also to the smaller and less active species of animals; but 
the rich places are generally mixtures of many plant and animal associations. 
3. THE PROBABLE EFFECT OF MAN’S INTERFERENCE ON THE 
DISTRIBUTION OF SPECIES. 
There are three phytogeographical regions in this country— 
(1) Arctic-Alpine Region— the sanctuary of plant formations trom 
former more widely extended arctic-tundra and alpine provinces. 
The region is very limited at present, and the plant associations 
chiefly migratory. 
(2) Moorland Region—the stronghold of plant formations isolated 
_ through the influence of an acid humus barrier. 
(3) Mesophytic Forest Region—occupying most of the area where the 
above humus barrier is ineffective or wanting in the Mesophytic 
Forest Province. 
(4) A Steppe Region is wanting in Britain (3). 
The stable or regional types of vegetation have overlapped on to the same 
areas during postglacial times, and this is believed (5) (4) (3) to be due to 
repeated secular shiftings in the boundaries of the climatic provinces. A 
