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igor] GENETIC DEVELOPMENT OF FORESTS 299 
the vegetation increases there is a gradually increasing incre- 
ment of humus, and this constitutes a condition favorable to a 
higher type of plant society. 
THE CONIFEROUS FOREST. It is not always easy to determine 
the conifer that first comes to occupy a place in the heath. If 
conclusions can be based upon a limited number of observations, 
the jack pine (Pinus Banksiana) is the most xerophytic (fig. 2). 
This is followed closely by the red pine (P. vesinosa) and the white 
pine (P. Strobus) in the order named. This does not mean that 
anyone of these always appears to the exclusion of all others, 
though that is sometimes the case (fig. 2). Other things being 
equal, the more xerophytic the conditions the more likely is it 
that the jack pine will be the predominant tree, and the less 
xerophytic the conditions the more abundant the white pine. 
fog. 3 is a photograph of a pine forest in which the red pines 
form a large proportion of the trees. In this forest are a num- 
ber of white and jack pines. If the succession as given above 
be correct, it means that this forest is probably a transition 
between the jack pine society and a white pine society. In the 
ideal case, then, the succession of predominant species is in the 
order named. The drier and perhaps the colder the climate up 
to a certain point, the more likely is this succession to be real- 
ized. Thus, in the Marquette region almost pure jack pine for- 
ests are found, while at North Manitou island, where the climate 
is more uniform, the jack pine stage is almost eliminated. Very 
often the balsam (Abies balsamea) (fig. 1) and hemlock ( Zsuga 
Canadensis ) occupy a prominent place in the coniferous forest. 
The first stages of the pine forest society are seen in the appear- 
ance of a number of trees in the later stages of the heath. For 
the first time in the normal development of the plant societies 
the tree has to be taken into consideration. In the tension zone 
between the heath and forest the trees are more scattered near the 
heath, and become gradually thicker as the forest is approached. 
The territory between the trees, where not shaded, is usually occu- 
pied by a growth of the heath plants already named. Thus, 
gradually the pines advance on the heath, and in time come to 
