80 



THE DEPTHS OF THE SEA. 



[CHAP. ii. 



an hour's sail may be sufficient to pass from the 

 extreme of heat to the extreme of cold. 



Finally, it had been shown that a large proportion 

 of the forms living at great depths in the sea belong 

 to species hitherto unknown, and that thus a new 

 field of boundless extent and great interest is open 

 to the naturalist. It had been further shown that 

 many of these deep-sea animals are specifically identi- 

 cal with tertiary fossils hitherto believed to be extinct, 

 while others associate themselves with and illustrate 

 extinct groups of the fauna of more remote periods ; 

 as, for example, the vitreous sponges illustrate and 

 unriddle the ventriculites of the chalk. 



THOIISHAVX. 



