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 a new 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 make 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 those northern 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 



