OF SOME SPECIES AT THE EDGES OF THEIR RANGES 49 
Changes of this character in the past are responsible for profound 
modifications of present phytogeography, such as the extinction 
of many types of conifers in the northern hemisphere. ‘There is 
no reason for supposing that evolution has ceased or that its 
progress is so slow as to make its effect on plant competition un- 
observable. (3) We know very little about the rapidity of 
climatic changes but climatic revolutions in the past have wrought 
great changes in vegetation. It is hardly reasonable to suppose 
that such changes are not still going forward. It may be doubtful, 
perhaps, whether they are rapid enough to be discernible in the 
present day flora, but it is clear that exceedingly slow movements 
of vegetation may be readily detected by observations of the ten- 
sion zones between the ranges. 
Broad conclusions involving a systematic shifting of plant 
ranges cannot of course be drawn from study of a single area. 
But the behavior of the species reaching the edges of their ranges 
in the Sugar Grove area seems to indicate very clearly that the 
ranges terminating there are not fixed but are changing. Plants 
of boreal affinity are apparently being displaced by others from 
the west and south. This obviously falls into line with the biotic 
changes which are known to have occurred since the glacial period. 
The conditions of the tension zones between the species terminating 
at Sugar Grove find, therefore, their most rational interpretation 
as a present-day continuation of the floristic movements following 
the glacial period. 
31 D 1913, reported that the contort and altitudinal extension of the lowland 
flora in the vicinity of the Desert Botanical Laboratory is checked by winter tem- 
peratures. This would of course fall into line an other facts, such as extremely 
localized ranges and the rich development of endemic forms, which indicate for the 
flora of the southwest a great age as compared with the comparative youth of the 
flora of the northeastern states. 
