60 THE PROBLEMS OF THE DEEP SEA 11 



would pass through one continuous mass of 

 Gloligerina mud, first of modern, then of tertiary, 

 and then of mesozoic date; the "chalks" of 

 different depths and ages being distinguished 

 merely by the different forms of other organisms 

 associated with the Globigerince. 



On the other hand, I think it must be admitted 

 that a belief in the continuity of the modern with 

 the ancient chalk has nothing to do with the 

 proposition that we can, in any sense whatever, 

 be said to be still living in the Cretaceous epoch. 

 When the Challenger's trawl brings up an Ich- 

 thyosaurus, along with a few living specimens 

 of Belemnites and Turrilites, it may be admitted 

 that she has come upon a cretaceous " outlier." 

 A geological period is characterized not only 

 by the presence of those creatures which lived 

 in it, but by the absence of those which have 

 only come into existence later; and, however 

 large a proportion of true cretaceous forms may 

 be discovered in the deep sea, the modern types 

 associated with them must be abolished before 

 the Fauna, as a whole, could, with any propriety, 

 be termed Cretaceous. 



I have now indicated some of the chief lines of 

 Biological inquiry, in which the Challenger has 

 special opportunities for doing good service, and 

 in following which she will be carrying out the 

 work already commenced by the Liyktning and 



