1918] HUTCHINSON—FOREST TREES 491 
have been instrumental in providing such conditions. As men- 
tioned before (under time factor), the time rate of change has been 
less in the lowlands and upon the rock outcrops of the Laurentian 
Plateau than upon the highland and the weathered glacial moraine. 
There is abundant evidence that Acer is migrating northward, its 
progress being contingent upon the time rate of soil development. 
FaGUS AMERICANA.—This species has a range of tolerance 
toward soil conditions which is even more limited than that of 
Acer. What has already been said for Acer applies equally for 
Fagus, since the latter is closely associated with the former. 
ULMus AMERICANA.—This species is another form whose dis- 
tribution defies explanation by considering either temperature 
or rainfall as limiting factors. The limit extends well into the 
plains and northward beyond Lake Winnipeg; it swings south- 
ward, then northward again in the region south of James Bay; 
then abruptly southward and again northward with no appar- 
ent dependence upon temperature or precipitation conditions. 
Even within its general limits it is found only where there is a clay, 
imperfectly drained soil; over large areas, especially throughout 
the Laurentian Plateau, it has not been found. ‘On the Mis- 
sinabi or west branch of the Moose River the white elm reappears 
130 miles north of its general boundary on descending to a sufh- 
ciently low elevation above the sea” (BELL 2). Soil conditions 
are the chief limiting factors; on the clay soil of the lowlands, where 
there is poor drainage, is its favorite habitat; for this reason it is 
intermittingly distributed. Its reappearance north of the height 
of land, its occurrence in the lowlands about Lake Winnipeg, as 
well as many other eccentricities of this species, may be explained 
upon this basis. 
BETULA LUTEA.—This species may be associated with pioneer 
forms such as B. papyrifera or climax forms such as Acer. “Yellow 
birch is the most abundant hardwood in New England” (12), 
while in the lake region it is seldom seen; it becomes more abun- 
dant in the Laurentian region. “It grows in forests of widely 
different composition and shares to some extent the habits of paper 
birch, appearing on burns in small even-aged stands”’ (12). The 
seedlings are pioneer, yet, because of its comparative longevity, 
