186 THE OCEAN 



fauna from remote geological periods which 

 some naturalists once believed might be 

 captured in the deep sea has yet to be dis- 

 covered. It is true that Discina and other 

 brachiopods, some of the irregular echinids, 

 and some of the siliceous sponges represent 

 ancient groups, but it is probable that among 

 the shore and fresh-water forms there are 

 representatives of faunas older than anything 

 to be found in the deep sea. 



Bipolarity. — As early as 1847 Sir James 

 Clark Ross noted that several Arctic species 

 occurred also in the Antarctic waters. Charles 

 Darwin in his " Origin of Species " mentions 

 J. D. Dana, J. Richardson and Joseph Hooker 

 as having observed resemblances between the 

 genera of the two polar regions, and Edward 

 Forbes gives some examples of generic forms 

 with two centres of dispersion, one in each 

 hemisphere, separated by a tropical dis- 

 continuity. 



In the first shallow-water dredgings in the 

 southern temperate regions the " Challenger " 

 naturalists were struck with the resemblance 

 of the fauna to what they were accustomed 

 to dredge off the coasts of Europe in similar 

 depths. C. Wyville Thomson writes : " These 

 shallow-water dredgings around Tristan da 

 Cunha gave a great amount of material, the 

 fauna being very much of the same character 



