Development of the Vegetation of New York State 49 
mental vigor. Thus the winter is not a recurrent catastrophe 
for our vegetation but a stimulus to it. Indeed cases are 
easily cited where a period of low or even freezing tempera- 
tures is necessary to stimulate germination of spores and 
seeds, growth of bulbs and tubers, bud opening, ete. It seems 
however, necessary, in spite of this apparent zonal fixity of 
species, to think of them as charged with the sort of energy — 
variability, vigor of growth, vigor of seed, resistance to 
disease, ete.-— which drives them toward the occupation of 
environments different from what they now hold, in short to 
burst beyond their zonal boundaries or rather to enlarge these. 
If this were not true, then we should have to believe that in 
the long run plants could not retain possession of the earth’s 
surface. So we think of sweet gum, tulip, oaks, hickories, 
as exerting a pressure of distribution to the north of territory 
now dominated by them; so with respect to maple, beech, 
birch, etc. Also we think of them as exerting pressure of 
distribution southward. 
Now it may well be —is undoubtedly — the case that a 
species may through age or whatever, become so fixed in its 
adjustment to a given environment as to have left little vigor 
of invasion — little competition power — or capacity to fol- 
low environmental changes; not even be able to resist the 
more vigorous species from invading its own zone. Possibly 
our conifers or some of them are in this condition. They are 
certainly very old species, not capable of much variation, 
not very vigorous in gaining new ground. Such species may 
be looked upon as on the road to extinction. We should 
expect such species from noting how whole dominant groups 
have disappeared from the earth. There are some to be 
found at the present —e. g., ceyeads — scattered as isolated 
genera of few species in widely different regions of the earth. 
It is of advantage to recognize these qualities of our 
species, both native and cultivated. Vigor, adaptability, 
resistance to disease and to aggression of other species; 
relation to climate and soil; longevity; rapidity of growth 
and development; abundance of seed production or vigor 
