PREFACE. Vil 
exact depth, in taking bottom and intermediate temperatures, 
and in determining the distribution of the deep-sea fauna by 
means of the dredge; and I have stated the general conclu- 
sions on various questions to which we were at that time led 
by the study and generalization of our results. Although the 
additional experience which we have now gained has caused 
me to alter my views on one or two important points, I adhere 
in the main to the opinions and statements contained in that 
book, and, to save unnecessary repetition, I shall regard “ The 
Depths of the Sea” as a general introduction to the series of 
volumes giving an outline of the Challenger Cireumnavigating 
Expedition. 
This last undertaking was on a very different scale from the 
trial cruises of the gun-boats, and the first chapter is occupied 
with the description of the much more complete arrangements 
for scientific work on board the Challenger. 
After the termination of the cruise of the Porcupine, I had 
a very strong conviction that the foraminifera of the genera 
Globigerina, Orbulina, and Pulvinulina, which are chiefly con- 
cerned in forming the modern chalk, lived on the bottom. Our 
later observations have, however, satisfied me that they never 
live on the bottom, but that they inhabit the surface, and the 
water to a limited depth beneath it. 
My general views with regard to ocean circulation remain 
unaltered. I think, however, that we have now good reason to 
believe that the indraught of water at a low temperature into 
the Atlantic and Pacific—gulfs, as we may almost call them— 
from the Southern Sea, is to a great extent due to an excess of 
precipitation over evaporation in the “ water-hemisphere ” and 
a corresponding excess of evaporation over precipitation in the 
“Jand-hemisphere ;” that, in fact, a part of the cireuit of general 
ocean circulation passes through the atmosphere. 
