48 College of Forestry 
set forth the plant organism as having the capacity to invade 
unoccupied territory. Vegetation was to be regarded by us 
as the forward march of plant life in its occupation of the 
earth. We did not raise the question as to when or how 
plant life began to occupy the earth. We simply opened the 
record at a period when we found evidence of a uniformly 
tropical earth and of a massive tropical vegetation on at 
least some of the land pretty much over the whole globe. But 
as ages pass, the earth becomes a different earth so far as 
offering a plant environment is concerned. Its surface under- 
goes momentous changes. Its climate changes even more 
fundamentally. Yet we find plant life keeping up with the 
environment as one may say. Thus in keeping pace with 
temperature changes we have these antipodal conditions 
where, on the one hand, a species carries out its vital pro- 
cesses of nutrition, growth, development, reproduction, in 
what amounts to hothouse climate the year round while on 
the other hand a species carries out its life functions at tem- 
peratures from near freezing to summer heat, with daily 
fluctuations of temperature from below freezing to summer 
heat, and during nine months of the year is held dormant by 
freezing temperatures which fall as low as sixty, seventy and 
even ninety degrees below zero Fahrenheit, as in forest 
regions in Arctic Siberia.* 
Now, in the long run, these temperature adjustments be- 
come so deeply impressed in the lying substance or pro- 
toplasm of a species that its zonal relation is a sort of fixed 
habit. Thus the species which in the long run dominate our 
New York vegetation — tulip tree, oak, chestnut, hickory, 
maples, beech, birches, hemlock, white pine, spruce, balsam, 
and so on, may be said to have become so thoroughly identi- 
fied with a climate which offers the sort of growing season 
and dormant season which New York does that the experi- 
ence of winter is a necessary experience to them. It is, of 
course, a phase of environment to be endured, but on the 
other hand it is a necessary means of stimulating develop- 
1 Warming, Ecological Plant Geography, p. 24, 
