E. V. Wulff —168— Historical Plant Geography 



those changes in the surface of the continents, particularly climatic 

 changes, induced by the shifting of the position of the continents as a 

 result of this oscillatory motion. 



Every animal and plant, in the process of multipHcation, naturally 

 tends to spread within the limits of the same cHmatic zone within 

 which it arose. Consequently, in the absence of obstacles to such dis- 

 tribution, their areas should encircle the globe in a band covering the 

 territory occupied by the given climatic zone. This regularity in the 

 distribution of organisms is upset by the oscillation of the earth. 

 Thus, a plant or animal finding itself in the region of the outer arc of 

 oscillation will be mechanically evicted from the climatic conditions to 

 which it has been accustomed and will be forced to migrate to the 

 west or east in order to return to its normal habitat conditions. This 

 serves as an explanation of those discontinuous areas consisting of two 

 halves symmetrically located on opposite sides of the arc of oscillation 

 (" symmetrische Punkte" in Simroth's terminology). Moreover, if a 

 species, as a result of such a change in habitat, does not undergo, in 

 the process of adaptation to the new habitat conditions, vital changes 

 in its morphological structure but acquires only a few, slight modifi- 

 cations, we shall find at these two points vicarious species. As an 

 example of such "symmetrical points", we may take Japan and Cali- 

 fornia. In case of the distribution of organisms farther inland in 

 western North America and eastern Asia within the limits of the same 

 latitudinal zone, we may speak of the horizontal symmetry of the dis- 

 tribution. This type of symmetry occurs chiefly during the polar 

 phase of the oscillation. During its equatorial phase, on the other 

 hand, the habitats shift to a very hot climatic zone, compelling marine 

 animals, in order to attain more temperate climatic conditions, to go 

 farther down into the sea and to adapt themselves to deep-water con- 

 ditions, while terrestrial animals must ascend the mountains or migrate 

 to the north or south, i.e., in a meridional direction. As a result, we 

 have distribution characterized by meridional symmetry, which is of 

 most frequent occurrence in the outer arc of oscillation. 



SiMROTH based his theory chiefly on carefully elaborated data on 

 the geographical distribution of animals, but he also gave a number of 

 examples of plant distribution that hkewise agreed with his theory. 

 Such agreement is expressed, first of all, in the actual existence of 

 symmetry in the present-day distribution of certain species and in the 

 fact that there are data establishing the existence in former times of 

 habitats of these same species on corresponding parallels in the outer 

 arc of oscillation. We shall here cite the most characteristic examples 

 of those presented by Simroth and other investigators. 



Let us first take a few instances from gymnosperm distribution. 

 Out of fifteen species of the genus Gnetum seven grow in equatorial 

 America, i.e., in the region of the western "Schwingpole", one in 

 Africa, and one on islands of the Pacific, while the six remaining species 

 are grouped around the eastern "Schwingpole" in the eastern part of 

 the Indian Ocean. Consequently, there is no doubt that we have here 

 horizontal symmetry. 



In the genus Pinus of most interest is the section Taeda, which 

 comprises sixteen species. One group of these species grows in America 



