Chapter X —165 — Historical Causes 



great "lost continents" or land-bridges of one form or another con- 

 nected the continents now separated by oceans. On the basis of 

 geological and paleontological data for different geological periods there 

 have been postulated various connections between the continents, pre- 

 sumably later having sunk to the bottom of the sea and having been 

 replaced by the upheaval of other land-bridges connecting other bodies 

 of land. According to this theory, the distribution of land and sea has 

 undergone constant change during the long history of the earth. 



The complex conformation of these putative land-bridges clearly 

 testifies to the artificial character of the hypotheses resting upon their 

 probable role. This, together with the fact that there are a number 

 of geophysical and geological arguments against this theory, has made 

 it necessary to seek for new ways of explaining the former continuity 

 of the now-discontinuous areas of organisms, all the more since the 

 time when these land-bridges were supposed to exist is not always such 

 as to be able to account for such continuity. 



Even as regards biogeography, to which the creation of this cum- 

 brous theory was a concession, it far from solves all the incompre- 

 hensible moments in the distribution of organisms. In particular, it 

 leaves unclarified why plants in former geological periods grew in 

 regions outside the climatic zones in which their present habitats are 

 found, and also, which is very important, it gives no satisfactory ex- 

 planation of discontinuous areas. If identical or closely related species 

 are found on two continents separated by an ocean, we cannot explain 

 this discontinuity of area merely by assuming the former existence of an 

 intervening continent where the ocean now lies. It would Hkewise be 

 necessary to assume that over the entire extent of this great "lost 

 continent" there existed like ecological conditions, similar to those in 

 the outlying portions of the area of the given species, these portions 

 having been preserved in the form of isolated fragments of a once- 

 continuous area. This can hardly be regarded as an acceptable hypoth- 

 esis. 



2. Theory of the Permanence of Oceans and Continents. — Against the 

 theory of land-bridges, despite its partial fulfillment of the requirements 

 of paleontology and biogeography and its acceptance by many ge- 

 ologists, there were advanced, beginning with the middle of the past 

 century, a number of serious objections. On the basis of these objec- 

 tions a new theory was proposed, the theory of the permanence of 

 oceans and continents. Among biologists this theory found many 

 supporters, the most outstanding being Darwin and Wallace. The 

 arguments advanced against the existence in the past of great conti- 

 nents that subsequently subsided to the bottom of the sea are so 

 weighty that biogeographers cannot fail to take them seriously into 

 account. These arguments, in brief, are as follows: 



Upon the upheaval of continental masses in the area of our present 

 oceans the great mass of water that had been in these oceans would 

 have inundated all the continents, both old and new, except for the 

 highest mountain peaks. Thus, the very continental connections that 

 it was desired to create by the assumption of the existence of such 

 "lost continents" would not have existed. 



But if it be assumed that the position of the oceans and continents 



