180 THE STORY OF THE EARTH AND MAN. 



tending to cause the weaker parts of the earth's crnsi 

 alternately to rise and subside at regular intervals of 

 time. Herschel, Adhemar, and more recently Croll, 

 have directed attention to astronomical cycles supposed 

 to have important influences on the temperature of the 

 earth. Whether these or other changes may have 

 acted on the equilibrium of its crust is a question well 

 worthy of attention, as its solution might give us an 

 astronomical measure of geological time. This question , 

 however, the geologist must refer to the astronomer. 



There are two notes of caution which must here be 

 given to the reader. First, it is not intended to apply 

 the doctrine of continental oscillations to the great 

 oceanic areas. Whether they became shallower or 

 deeper, their conditions would be different from those 

 which occurred in the great shallow plateaus, and these 

 conditions are little known to us. Further, throughout 

 the Palaeozoic period, the oscillations do not seem to 

 have been sufficient to reverse the positions of the 

 oceans and continents. Secondly, it is not meant to 

 affirm that the great Permian plications were so wide- 

 spread in their effects as to produce a universal de- 

 struction of life. On the contrary, after they had 

 occurred, remnants of the Carboniferous fauna still 

 flourished even on the surfaces of the continents, and 

 possibly the inhabitants of the deep ocean were little 

 affected by these great movements. True it is that the 

 life of the Palaeozoic terminates with the Permian, but 

 not by a great and cataclysmic overthrow. 



Wo know something at least of the general laws of 



