1918] HUTCHINSON—FOREST TREES 479 
are able to keep pace, and where ecological changes are more rapid 
species having the best methods of seed dispersal naturally migrate 
most rapidly. The rapid invasion of a burned area by the Populus- 
Betula association is due primarily to the widespread dispersal of 
the seeds of these species. In contrast, Pinus takes its. place 
among the trees which appear later, largely because it has a less 
efficient method of scattering seeds. A number of examples of the 
limitations of seed dispersal have been noted. In several cases 
where a burn had left only one or two pines upon an island the 
usual Populus-Betula association was unable to gain a foothold 
because of the distance from the mainland; hence these species were 
superseded by numerous pine seedlings. Doubtless the same prin- 
ciples may be applied to the relation between seed dispersal and 
tree migration even over greater areas. The time factor of distri- 
bution may be an expression of the rate of migration as it is deter- 
‘mined by the method of seed dispersal. 
The time factor of distribution is an expression of the rate of 
migration. The rate of migration is dependent upon such primary 
conditions as temperature, water supply, soil properties, light in- 
tensity, and methods of distribution. Time, as a condition of 
change in environmental factors, becomes itself of great importance 
in any consideration of the factors of forest distribution. 
Competition factor 
Competition results in the survival of the fittest. The fittest 
is that species or individual whose specific range of tolerance is best 
related to the environmental condition acting as a limiting factor 
toward other species; hence temperature, water supply, soil, or 
light may act as the basis of competition. Time may also act as a 
basis of competition, since it changes conditions in environmental 
factors. In order that competition may act as a distributional 
factor, conditions must be favorable for one or more species and 
unfavorable for others. While the time factor is an expression of 
the rate of change of the environmental factor acting in a limiting 
Capacity, the competition factor is an expression of the relation 
between the ranges of tolerance of the forms in question toward 
the limiting environmental factor. 
