Chapter X —167— Historical Causes 



Australia and Asia, Madagascar and Africa, the Antarctic continent 

 and South America (Soergel, 191 7). But this concession to bio- 

 geography is not founded on any new facts. Moreover, conceding the 

 existence of such narrow land-bridges does not, by any means, solve all 

 the unexplained problems in the distribution of organisms; it is neces- 

 sary, in order to explain the latter, to presume still other connections 

 not compatible with the theory of permanence. From this vicious 

 circle these early theories provide no exit. An exit is found, in our 

 opinion, only in Wegener's theory of continental drift, which we shall 

 later discuss in detail. 



4. Pendulum TJieory.—An attempt to reconcile the contradictions 

 involved in the two theories above outlined was made by the advocates 

 of the so-called "pendulum theory", that aimed also to explain, on the 

 basis of climatic changes in former geological periods, those cases of 

 plant distribution that are not at all in accord with present-day cli- 

 matic zones. 



Changes in the position of the climatic zones on the globe— attested 

 to by data on the distribution of fossil plants, showing, for instance, 

 that at one time there was a rich flora within the area of the present 

 Arctic region — must presumably have been caused not by a different 

 location of the sun in relation to the earth but by a different location 

 of the continents in relation to the sun. The pendulum theory ex- 

 plains these phenomena by assuming that periodic changes have oc- 

 curred in the position of the poles caused by their oscillating back and 

 forth like a pendulum. This theory was first advanced by the geologist 

 Reibisch and later elaborated on the basis of biogeographical data by 

 SiMROTH (1914). These investigators start from the assumption that 

 the earth, besides an axis of rotation and poles of rotation located at 

 the north and south ends of this axis, has an axis on which it oscillates 

 like a pendulum. The two poles of this latter axis— known, in Sim- 

 roth's terminology, as " Schwingpolen "— are located one in Ecuador 

 and the other in Sumatra. Not subject to the oscillatory motion are 

 only this axis and its two poles, Ecuador and Sumatra. These points 

 alone remain fixed and under constant tropical conditions, while all 

 the other points on the earth's surface are subject to periodic changes 

 in climatic conditions induced by the oscillatory motion. Moreover, 

 the degree of these climatic changes is determined by the distance 

 from the equator. The greater this distance, the greater the devi- 

 ations in climate suffered by any given point on the earth's surface. 

 Conversely, those regions of the earth nearest to the equator, partic- 

 ularly those nearest to the "Schwingpolen", are subject io the least 

 changes, chmatic and physiographic, and, consequently, in these re- 

 gions ancient plant and animal forms have been preserved to a much 

 greater extent than in those regions located on the outer arc of oscilla- 

 tion. 



If the pendulum theory is accepted, there is no need for the theory 

 of land-bridges, since changes in the level of the sea induced by this 

 oscillation of the earth would suffice to cause the joining together and 

 disjoining of different parts of the earth's land surface and, moreover, 

 in such a way as serves to explain the distribution of plants and ani- 

 mals. Simroth's theory of the distribution of organisms is based on 



