VEGETATION TYPES. 
137 
For example, although the percentages for single species might not 
be very accurate, when those of all the species belonging to a certain 
family are consolidated, the errors are bound to counterbalance each 
other to a considerable extent. It is useful to remember that the 
figures showing the percentages of different species in a given area 
or region (if not too small) when arranged in order tend to form 
a geometrical progression, those near the head of the list always 
being farther apart than those near the foot. In other words, there 
are everywhere more rare species than abundant ones. 
The separation of trees, shrubs, herbs, etc., has several ad¬ 
vantages, one of which is that it is not practicable to compare the 
abundance of species differing greatly in size. The trees too are 
more reliable indicators of environmental conditions than the smaller 
plants are, because their roots penetrate a greater variety of soil and 
their branches reach higher into the air, and because owing to their 
longer life they have to withstand the climate of all seasons and of 
many different years. The occurrence of many herbs depends on 
the shade and humus furnished by the trees around them more than 
it does on the mineral soil underneath; and the cold of winter, when 
they are dead, makes little difference to them; but most of the trees 
have to have the right kind of soil in the beginning, and there is a 
certain minimum endurable temperature for each one. 
This study is concerned only with native plants, and it seeks to 
reconstruct the primeval conditions as far as possible, disregarding 
the numerous introduced weeds that flourish in fields and lots, along 
roads and railroads, etc., as well as all cultivated crops. 
' ■ • 
DETAILS OF TREATMENT. 
For each type of vegetation the more obvious characters of soil, 
topography and ground-water are first indicated, with references 
to the chapter on soils for more detailed information. Under the 
head of soil are included some desultory observations on the soil 
fauna, a very important matter which has been strangely neglected 
by most soil investigators. Allusion is made also to the frequency 
of fire, an environmental factor of great importance which has re¬ 
ceived very little consideration from botanists, especially those who 
do most of their research in laboratories ( for fire does not lend itself 
well to experimental treatment). Although fire is a secondary fac¬ 
tor, depending largely on the vegetation itself, there are many places 
where physiographic factors reduce its frequency to a minimum, and 
. . • ■ 
