CHAP, v.] GENERAL CONCLUSIONS. 283 



vast areas where the calcareous matter of the pelagic foraminif- 

 era has been removed, and the bottom consists of red or gray 

 clay, animal life is scarce ; and is represented chiefly by shell- 

 less orders, such as the Ilolothuridea and the Annelids. This 

 comparative sterility depends, no doubt, to a great degree upon 

 the absence of carbonate of lime, but not entirely so ; for the 

 most sterile regions of the whole sea are the mortar-like lime 

 deposits which form the slopes of coral reefs and islands. There 

 appears to be something in the state of aggregation of the lime 

 in the Globigerina shells and its intimate union with organic 

 matter which renders the globigerina ooze a medium peculiarly 

 favorable to the development of the higher forms of life. The 

 stomachs of the more highly organized animals living in it or 

 on its surface are always full of the fresher f oraminiferal shells, 

 from which they undoubtedly derive not only material for the 

 calcification of their tests, but nitrogenous matter for assimila- 

 tion likewise. 



As we had previously anticipated, the fauna at great depths 

 was found to be remarkably uniform. Species nearly allied 

 to those found in shallow water of many familiar genera were 

 taken in the deepest hauls, so that it would seem that the enor- 

 mous pressure, the utter darkness, and the differences in the 

 chemical and physical conditions of the water, and in the pro- 

 portions of its contained gases depending upon such extreme 

 conditions, do not influence animal life to any great extent. 



The geographical extension of any animal species, whether 

 on land or in the sea, appears to depend mainly upon the main- 

 tenance of a tolerably uniform temperature, and the presence 

 of an adequate supply of suitable food, the latter condition 

 again depending chiefly upon the former; and the conditions 

 both of temperature and of food-supply are very uniform at 

 extreme depths where the nature of the bottom is the same. 

 Possibly the element next in importance is the length of time 

 during which migration may have taken place, and there seems 



