ON A PIECE OF CHALK 55 



vegetable organisms which are called Diatomacece, 

 and partly to the minute, and extremely simple, ani- 

 mals, termed Radiolaria. It is quite certain that these 

 creatures do not live at the bottom of the ocean, but 

 at its surface where they may be obtained in pro- 

 digious 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 thou- 

 sand feet of water, before they reached their final rest- 

 ing-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 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 

 Globigerinos, in proportion to other organisms, of like 

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

 deep-water Globigerinas are larger than those which 

 live in shallower parts of the sea; and such facts nega- 



