GEOGRAPHICAL DISTRIBUTION 
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city to suit other climates, is largely practised, and with 
success in many cases, especially if the change be not too 
marked. 
Physical Barriers. Of the topographical agents in 
determining the distribution of species, the natural physical 
barriers are very important, e.g. seas, oceans, deserts, moun- 
tain-chains, &c. At the same time many plants are able to 
pass over wide expanses of water or over ranges of moun- 
tains (p. 109); these however are but few in proportion to the 
whole flora, and generally the floras on opposite sides of a 
great natural barrier are almost completely different. Ranges 
of mountains also act as pathways of migration by reason of 
the new soil frequently exposed in landslips or otherwise ; 
thus species with good dispersal mechanisms may travel 
along the whole range and get into new countries. 
* The actual elevation of the mountains has also an 
important effect, for the temperature decreases with increase 
of elevation, while the humidity often increases, and the air 
is less dense. The steepness of slopes, and their exposure, 
are also important factors, regulating drainage and exposure 
to the sun ( insolation ). In Europe a southern slope is 
warmer than a northern, in the tropics an eastern than a 
western, because the sky is usually clearer in the morning. 
Soil. The nature of the soil has a great effect upon the 
composition of the flora growing upon it ( edaphic influence). 
Probably the chemical composition of the soil produces 
more effect than the physical. Thus we have a group of 
plants which grow by preference on soils with much silica 
(sand), another on chalky soils, and a third on salt soil 
(halophytes, see below). The amount of water the soil is 
able to retain is a factor of much importance ; so also is 
the proportion of humus, for if there be very much only 
true saprophytes (see below) or plants with mycorhiza are 
able to do well on it. The depth of the soil and its con- 
ductivity to heat, and many other factors, also take part in 
the problem. 
Biological Agents are those causes of limitation of 
specific areas which depend upon the interaction of living 
organisms with one another. Parasites can only spread 
with their hosts, saprophytes only where there is plenty of 
suitable humus, i.e. usually only where there are woods of 
w. 
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