VARIATION BY ENVIRONMENT. 
241 
AYe often find three zones of plants in the same 
field if a part of it is swampy. In the lowest places 
broad-leaved plants thrive. Their leaves and stems 
are both adapted to a watery environment. On the 
dry ground we may find plants of an entirely differ¬ 
ent structure. Their stems are more slender and 
firmer in texture. The leaves are of a different 
shape, having most of the stomata on the under side. 
Both leaves and stems are often covered with down 
or bristles to protect them from injurious insects. 
Between this zone on high ground and the one on 
swampy ground is another in which the plants have 
an intermediate structure, so that they are adapted, as 
far as possible, to both wet and dry environment, for 
part of the year this zone is well supplied with water, 
but is rather dry during the rest of the year. AVe 
have here then three different plant societies which 
are the result of environment. 
It is true that the plants of one zone grade more 
or less into those of another. If several dry seasons 
follow each other, the middle zone encroaches upon 
the aquatic, and the plants of the dry zone occupy 
some of the middle ground. Thus it is that climate 
to a large extent determines the character of vegeta¬ 
tion. 
How about animals ? 
Snails in England, Mr. Grant Allen tells us, 
that live near the water of the limestone region, have 
a calcareous shell of good proportions. On the con¬ 
trary, those confined to localities where the surface is 
covered with mold have either no shell or only a 
