46 THE VOYAGE OF H.M.S. CHALLENGEE. 



fossil from the Clialk ; and a remarkable group of irregular urchins, clustering round the 

 genus Pourtalesia, and recalling such cretaceous forms as Infulaster, Holaster, Micraster, 

 and Anancliijtes. The curious group of Holothuridea, the Elasmopoda, to which I have 

 already referred. A large family of Crustacea having a strong general resemblance to 

 the fossil genus Eryon (fig. 22), and a group of gigantic schizopods, such as Petaloph- 

 ihalmus and Giiathopliausia. The fishes include many marvellous and grotesque species, 

 chiefly referable to the Ophidiidse, the Macruridse, and the Scopelidse (fig. 24). 



We have already seen that the fauna of shallow water is roughly mapped out into 

 zoological areas ; resembling, and to a certain extent following the land in this respect. 

 A fauna so uniform and so continuous as that of the abyssal region cannot be directly 

 derived from the faunae of the marine Provinces, which form its littoral fringe, even 

 although these are somewhat laxly defined. It has all the appearance of having been 

 derived from a genetic source dating much further back than the minor oscillations which 

 have from time to time during the latest geological periods produced great changes 

 of level in the land, sufiicient to raise effective barriers to distribution, and to produce 

 local changes of climate and changes of level in the sea-bottom to a considerable, if to a 

 much more limited, degree. 



The question therefore remains to be solved, and it is one of the highest interest, 

 whence, and from what genetic source is this uniform and apparently independent fauna 

 derived ? 



The Source of the Abyssal Fauna. — I suppose I am now entitled to regard the view 

 as widely accepted by geologists, that the age of the most obvious depressions in 

 the crust of the earth, which are now filled by the sea, is much greater than we were at 

 one time led to believe. I long ago expressed the opinion that the primary meridional 

 grooves of the earth's crust dated from its original cooling; whether this be so or not, 

 there seems to be suflficient evidence that all changes of level since the close of the 

 Palaeozoic period are in direct relation to the present coast-lines. 



There does not seem to be a shadow of reason for supposing that the gently undu- 

 lating plains, extending for over a hundred millions of square miles at a depth of 2500 

 fathoms beneath the surface of the sea, and presenting like the land their local areas of 

 secular elevation or depression, and their centres of more active volcanic disturbance, 

 were ever raised, at all events in mass, above the level of the sea ; such an arrangement, 

 indeed, is inconceivable. If, then, such a condition did not at any time exist, a con- 

 tinuous ocean must always have extended over the greater part of the earth's surface, 

 and must have occupied continuously any secular areas of depression due to the assumption 

 by the world of its present physical features. 



Without entering into the vexed question of recurrent glacial epochs, there is 

 certainly no evidence from palaeontology that the temperature conditions of the sea at 



