II THE PROBLEMS OF THE DEEP SEA 57 



pated ; aiid, probably, further investigation will largely add to 

 this class of data, and will give us an opportunity of testing 

 our determinations of the zoological position of some fossil 

 types by an examination of the soft parts of their recent 

 representatives. The main cause of the destruction, the migra- 

 tion, and the extreme modification of animal types, appear to 

 be change of climate, chiefly depending upon oscillations of the 

 earth's crust. These oscillations do not appear to have ranged, 

 in the Northern portion of the Northern Hemisphere, much 

 beyond 1,000 feet since the commencement of the Tertiary 

 Epoch. The temperature of deep waters seems to be constant 

 for all latitudes at 39 ; so that an immense area of the North 

 Atlantic must have had its conditions unaffected by tertiary or 

 post-tertiary oscillations. " 1 



As we shall see, the assumption that the tem- 

 perature of the deep sea is everywhere 39 F. (4 

 Cent.) is an error, which Dr. Wyville Thomson 

 adopted from eminent physical writers; but the 

 general justice of the reasoning is not affected by 

 this circumstance, and Dr. Thomson's expectation 

 has been, to some extent, already verified. 



Thus besides Globigerina, there are eighteen 

 species of deep-sea Foraminifera identical with 

 species found in the chalk. Imbedded in the 

 chalky mud of the deep sea, in many locali- 

 ties, are innumerable cup-shaped sponges, pro- 

 vided with six-rayed silicious spicula, so disposed 

 that the wall of the cup is formed of a 

 lacework of flinty thread. Not less abundant, 

 in some parts of the chalk formation, are the 

 fossils known as Ventriculites, well described by 



1 The Depths of the Sea, pp. 51-52. 



