ii8 MOSTLY MAMMALS 



there are few instances where the swing of the pendulum 

 of opinion to one side has been more swiftly followed by 

 its oscillation to the other than has been the case in the 

 problem of the permanency of continents and oceans. 

 When geology first began to take rank among the exact 

 sciences, and it was demonstrated that most of the shells 

 and other fossils found in the solid rocks of many of 

 our continents and islands were of marine origin, it was a 

 natural, if hasty, conclusion that land and sea had been 

 perpetually changing places, and that what is now the 

 centre of a continent might comparatively recently have 

 been an ocean abyss. Accordingly, when any difficulty 

 in finding an adequate explanation in regard to the 

 geographical distribution of the animals or plants of two 

 or more continents or islands occurred, the aid of an 

 " Atlantis " or a " Lemuria " was at once invoked without 

 misgiving, and a path thus indicated across which the 

 inhabitants of one isolated area could easily have passed 

 to another. 



This was one swing of the pendulum. But as the 

 methods of geological observation and investigation became 

 more exact and critical, it was soon obvious that, in many 

 areas at least, the alternations between sea and land could 

 not have been so frequent or so general as had been at 

 first supposed. It was, indeed, perfectly true that many 

 portions of some of our present continents had for long 

 periods been submerged, or had been at intervals alter- 

 nately land and sea. But at the same time it began to 

 be realised that the fossihferous marine deposits commonly 

 met with on continents and large islands were not of such 

 a nature that they could have been laid down in depths 

 at all comparable to those now existing in certain parts of 

 the basin of the Atlantic. Even a formation like our 



