302 THE ATLANTIC. [chap. v. 



6. The abyssal fauna is certainly more nearly related than the 

 fauna of shallower water to the faunae of the Tertiary and Sec- 

 ondary periods, although this relation is not so close as we were 

 at first inclined to expect, and only a comparatively small num- 

 ber of types supposed to have become extinct have yet been 

 discovered. 



7. The most characteristic abyssal forms, and those which are 

 most nearly related to extinct types, seem to occur in greatest 

 abundance and of largest size in the Southern Ocean ; and the 

 general character of the faunae of the Atlantic and of the Pa- 

 cific gives the impression that the migration of species has taken 

 place in a northerly direction, that is to sa}^ in a direction cor- 

 responding with the movement of the cold under-current. 



8. The general character of the abyssal fauna resembles most 

 that of the shallower water of high northern and southern lati- 

 tudes, no doubt because the conditions of temperature, on which 

 the distribution of animals mainly depends, are nearly similar. 



The Density of Sea-water. — The specific gravity of the sur- 

 face-water was determined daily by Mr. J. Y. Buchanan, the 

 chemist to the expedition, with great accuracy ; the specific 

 gravity of the bottom- water was also determined so far as pos- 

 sible at every observing station, and every opportunity was 

 taken to procure for phj^sical and chemical examination sam- 

 ples of water from intermediate depths. On our return home 

 through the Pacific, Mr. Buchanan, at my request, prej)ared a 

 preliminary report on his method of investigation and on the 

 general results of his work, which I received at Valparaiso ; 

 and from that report the following summary of specific-gravity 

 conditions in the Atlantic, according to the first year's obser- 

 vations, is taken. The apparatus in use for procuring water 

 from the bottom and from intermediate depths has been already 

 described (vol. i., p. 51 et seq.). 



Representing the specific gravity of distilled M'ater at 4° C. 

 by 100,000, Mr. Buchanan found that of ocean-w^ater at 15°-56 



