ix.] ON A PIECE OF CHALK. 185 



numbers by the use of a properly constructed net- 

 Hence it follows that these silicious organisms, though 

 they are not heavier than the lightest dust, must have 

 fallen, in some cases, through fifteen thousand feet of 

 water, before they reached their final resting-place on 

 the ocean floor. And, considering how lame a surface 

 these bodies expose in proportion to their weight, it 

 is probable that they occupy a great length of time 

 in making their burial journey from the surface of the 

 Atlantic to the bottom. 



But if the Radiolaria and Diatoms arc thus rained 

 upon the bottom of the sea, from the superficial layer 

 of its waters in which they pass their lives, it is ob- 

 viously possible that the Globigerince may be similarly 

 derived ; and if they were so, it would be much more 

 easy to understand how they obtain their supply of food 

 than it is at present. Nevertheless, the positive and 

 negative evidence all points the other way. The 

 skeletons of the full-grown, deep-sea Globigerince are 

 so remarkably solid and heavy in proportion to their 

 surface as to seem little fitted for floating; and, as a 

 matter of fact, they are not to be found along with the 

 Diatoms and Radiolaria, in the uppermost stratum of 

 the open ocean. 



It has been observed, again, that the abundance of 

 Glohiy evince, in proportion to other organisms, of like 

 kind, increases with the depth of the sea ; and that 

 deep-water Globigcrince are larger than those which live 

 in shallower parts of the sea ; and such facts negative 

 the supposition that these organisms have been swept by 

 currents from the shallows into the deeps of the Atlantic. 



It therefore seems to be hardly doubtful that these 

 wonderful creatures live and die at the depths in which 

 they are found. 1 



1 During the cruise of H.M.S. Lull-dor/, commanded by Sir Leopold 



