I ON A PIECE OF CHALK 15 



obtained in prodigious 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 

 large 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 are thus 

 rained upon the bottom of the sea, from the 

 superficial layer of its waters in wliich they pass 

 their lives, it is obviously possible that the 

 Grlobigerina may be similarly derived ; and if they 

 were so, it would be much more easy to under 

 stand 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 Globigcrincc 

 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 

 Globiyerincc, in proportion to other organisms, of 

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



