48 GRIGGS: OBSERVATIONS ON THE BEHAVIOR 
occupy ranges as large as their adaptability to climate would 
permit, just as stream erosion would reduce all regions to base- 
level. Under such conditions competition between species would 
cease to be a factor, for that battle would have been finished 
and the weaker species eliminated. With competition would 
disappear the historical factor which is largely a record of the state 
of the struggle. 
At best, however, such conditions obtain only for part of the 
plants. There are always a large number of species like the cactus 
cited above which are driven out of the favorable habitats by more 
aggressive plants but find a place in unfavorable stiuations where 
their competitors cannot follow them but which they can endure, 
Thus the plants of favorable habitats may be limited by climatic 
factors while those of unfavorable are largely controlled by com- 
petition. Accordingly one may find a species dominant at one 
edge of its range but outcast at the other. As one passes from 
zone to zone into more and more severe conditions he may find 
the outcast plants of one region becoming the dominant ones of 
the next, giving an appearance of complete adjustment to climatic 
conditions whereas in reality only half the vegetation is controlled 
by climate. 
Where conditions have remained stable such adjustment should 
be found everywhere but as species approach the limits of their 
adaptability to climatic conditions their progress may be expected 
to be increasingly retarded so that as in base-leveling the last 
stages of the process are exceedingly slow. With our present 
limited knowledge of plant ranges it would be rash to suppose 
that complete climatic adjustment of vegetation is as rare as the 
completion of a cycle of erosion, but it is evident that various 
factors may intervene to disturb the process before completion. 
(1) The minor changes brought about by shifting physiography 
might affect the distribution of such plants as require certain 
peculiar habits. (2) Whenever and wherever new types of plants, 
or of animals that feed upon them, are evolved, their entry into 
the struggle for existence introduces an entirely different set of 
conditions and throws the whole vegetation* out of adjustment. 
* Shreve in a paper entitled “ The réle of winter temperatures in determining 
the distribution of plants,’’ read before the Botanical Society of America at Atlanta, 
