142 
GEOGRAPHICAL DISTRIBUTION 
Salsola, Mimosa, Gomphocarpus, Galinsoga, Elodea, which 
have been introduced by man into countries where they did 
not previously exist. Their rapid spread shows that in long 
periods any species, however poor its dispersal mechanism, 
might cover a large part of the earth, if no other cause 
prevented this result. How far the species spreads will 
evidently depend upon its general ecology ; it may be 
admirably adapted to the circumstances of its original 
environment, but when its dispersal mechanisms carry it 
into different climatic circumstances, or different associations 
of competing plants, it may be unable to hold its own. If 
very adaptable or very well equipped it may continue to 
spread, diverging into new species or varieties as it goes, till 
the genus thus formed may cover an immense area. The 
causes which limit the area occupied by a species are many. 
They may be divided into the geographical agents, those 
which vary with locality, e.g. light, heat and moisture, the 
topographical agents, e.g. mountains, rivers, seas, the consti- 
tution of the soil, &c., the biological agents, e.g. the distribu- 
tion of insects and other animals, the mutual competition of 
organisms, the dependence of one organism upon another, 
and so on. Last but not least, account has to be taken of 
the conclusions of geology with regard to alterations in the 
configuration and climate of the earth’s surface, for most of 
our existing species are descended from forms that existed 
under different environments. 
Light varies much in different latitudes. In the tropics 
the day is always about 12 hours long, the light nearly 
vertical at midday, while in the arctic regions there is 
long-continued darkness in winter, and almost continuous 
light in summer, falling from every side in turn as the sun 
describes its daily course. The amount of assimilation 
depends largely upon the amount and intensity of the light. 
Plants of high latitudes go through their vegetative period 
more rapidly than those of low\ Many plants require 
brilliant light for full success, others prefer weak light or 
shade ; this difference comes out in their structure, e.g. in 
the sun-plants the stems are often short, light retarding 
growth, the leaves often turned edgewise (p. 49), the 
palisade-tissue more developed. 
Heat also depends largely on the latitude, but shows 
