476 BOTANICAL GAZETTE [DECEMBER 
made necessary. To a greater or less extent this is true of all the 
species forming the forests of Ontario. Under existing tempera- 
tures any further migration is dependent upon changes in the . 
conditions now acting as limiting factors, as water, soil, and light. 
The rate of migration, and hence the distribution of forest trees, 
is dependent, primarily, upon the rate of change in temperature; 
however, migration may be restricted by other factors. 
There is reason to believe that the yearly precipitation has 
gradually decreased since the glacial epoch. The data regarding 
the exact extent of these changes are limited. There can be no 
doubt, however, that the westward migration made possible by 
temperature changes has been checked by the water factor; also the 
irregularity of the limits of Pinus Banksiana may be explained by 
the fact that although temperature conditions have so changed that 
this species has migrated to 56° N. lat. in the highlands of northern 
Quebec, it has been limited in its northward progress by the low 
lying lands south and westward from James Bay. The incon- 
sistencies of data regarding the northward distribution of Pinus 
Banksiana are doubtless due to the presence of certain outliers 
which might be expected when available soil moisture and other » 
soil conditions act as the limiting factor, but which would be most 
improbable were temperature the determining factor of distribu- 
tion. In regions where water is a limiting factor the rate of 
migration is dependent upon the rate of change in water condi- 
tions, in other words, upon the time factor. 
Time factor in relation to soil development 
It has been demonstrated that soil development, particularly 
with reference to the humus content, is a potent factor in deter- 
mining the boundaries separating the Acer-Fagus and the Abies- 
Picea forests of Ontario. Since the Acer-Fagus forest demands 
the most highly developed soil, we are forced to the conclusion 
that in a forest succession the deciduous hardwood forest is the 
climax type. Over a vast area this climax type of forest has been 
excluded by soil conditions rather than by temperature. North- 
ward migration of the deciduous hardwood forest has been limited 
by the rate of soil changes rather than by the rate of temperature 
