1918] _ HUTCHINSON—FOREST TREES 475 
factor in limiting the distribution of forest species is an expression 
of the rate of change in ecological conditions and of the specific 
_ rates of migration of the various species. Ordinarily conditions 
change so slowly that migration keeps pace; when there are more 
rapid changes migration lags behind. The time factor, therefore, 
must be considered in relation to the rate of change of such condi- 
tions as temperature, water, soil, light intensity, and secondarily in 
relation to methods of distribution. 
WARMING (24) states, ‘‘Changes in the physical relationship of 
the soil are everywhere and always taking place, and in close corre- 
lation with this plant communities also undergo modification, but 
it does not seem possible to use development as the fundamental 
basis of classification of plant societies.” (CowzrEs (7), while 
recognizing the same factors, attaches more importance to develop- 
ment of successional associations. 
The forests of Ontario have been made possible only by the 
retreat of glacial ice and water and the establishment of condi- 
tions permitting the growth of trees. It is evident that modifica- 
tions of temperature have been prerequisite for the northward 
migration of tree species; by many it is regarded as the only factor. 
ADAMS suggests that the northern migration following the retreat- 
ing glacier would comprise 3 great waves of life: (1) a wave of 
glacial or arctic vegetation, of which there are remnants in New 
York and Mount Marcy and two or three other high peaks; (2) a 
wave comprising the northernmost species of trees, stunted willows, 
birches, alders, and the coniferous forest spruces, hemlock, and 
pines; (3) a wave embracing the temperate zone deciduous trees. 
HARSHBERGER records a similar conclusion: ‘“‘Several great waves 
of plant migration may be recognized, namely, glacial vegetation, 
tundra coniferous forests, and a migration of the deciduous forest 
elements from the southeastern center.’ If forest migration has 
kept pace with temperature changes, it might be expected that 
the limits of forest species would conform in outline with respec- 
tive isotherms. It has already been demonstrated that in many 
instances this is not the case. The conclusion that in many places — 
the migration of such species as Tsuga canadensis, Acer saccharum, 
and Fagus americana has lagged behind temperature changes is 
