598 METASPERMAE OF THE MINNESOTA VALLEY. 
Again the equatorial pressure has an indirect influence upon 
habitat, under the law termed by Herbert Spencer the multipli- 
cation of effects. As one plant is forced into a new and gener- 
ally poorer habitat, to which it becomes more or less exactly 
accommodated, it exerts a constantly widening influence upon 
other plants some of which, already established in its new 
habitat, are brought into anew phase of the struggle for ex- 
istence by the recent addition, and others competing for the 
abandoned territory are in turn exposed to the modifying in- 
fluence of natural selection. Thus it happens that the general 
effect of what has been termed equatorial pressure has an in- 
calculably wide and profound influence upon the plant physi- 
ognomy of any district. In this analysis it will be seen that 
general answers—partial, it is true, but capable of extension— 
are provided for some of the questions propounded in the 
opening pages of this chapter. Conditions in the Minnesota 
valley must be explained by conditions elsewhere. This area 
in the line of tension must be studied with an eye directed 
towards the dynamic centers which nae it possible for such 
a line of tension to exist. 
Secondary longitudinal tensions. Besides the general line 
of tension to which notice has been directed there exist at least 
six other principal secondary longitudinal tension-lines in the 
North American continent. The influence of these is felt but 
slightly in the Minnesota valley, in comparison with the 
lateral line. The origin of the six principal longitudinal 
tensions is to be referred to the three meridianally extending 
mountain ranges that arise in the eastern, western-central 
and western regions of the continent. Between the Sierras 
and the Pacific coast occurs the western tension-line; between 
the Sierras and the Rockies what may be termed the Sierra 
and western Rocky mountain tension-lines; between the 
Rockies and the Appallachians, what may be termed the 
central and Appallachian tension-lines, and between the 
Appallachians and the Atlantic coast, the eastern tension -line, 
The origin of these tension-lines is precisely similar to that 
of the main continental tension-line that runs in a direction 
generally east and west. They arise from the fact that the 
alpine summits and elevations serve for southward extensions 
of the northern group of plants, and thesenorthern plants are 
brought into competition with the plants of lower levels which 
are crowded laterally as well as longitudinally, and tend to 
expand their areas of distribution from meridian to meridian 
