118 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 fossiliferous 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 
