80 THE ANATOMY OF IXVERTEBRATED ANIMALS. 



constantly occur at the surface of all temperate and tropical 

 seas, and, together with the HacUolaria and the diatoma- 

 ceous plants which accompany them, form an important in- 

 gredient in the food of pelagic animals, such as the Salpcp.. 



It is no less certain that, at all depths down to 2,400 fath- 

 oms or thereabouts, GlobigeriiicB i\\ all stages of growth, and 

 containing more or less protoplasmic matter, are found at the 

 bottom mixed with the cases of the surface Diatoms and the 

 skeletons of Itadiolaria. The proportion of Glohigerincp, 

 OrbuUnce, and Pulvinularice, in the deep-sea mud increases 

 with the depth until, at depths beyond 1,000 fathoms, the 

 sea-bottom is composed of a fine, chalky ooze made up of 

 little more than the remains of these Foraminifera and their 

 associated Diatoms and JRadiolaria. 



It may be regarded as certain, therefore, that some of the 

 chalky ooze arises from the precipitation to the bottom of the 

 skeletons of dead Glohigerinoe^ Pulmnularia?^ and Orbulince^ 

 and it may be that the whole has this origin. On the other 

 hand, it may be that a greater or smaller proportion of these 

 Foraminifera really live at the bottom, as their congeners 

 are known to do at less depths. 



It has been said that the condition of the surface-waters 

 and sea-bottom which has just been described obtains in all 

 temperate and hot seas ; or, speaking roughly, for 55° on 

 either side of the equator. Toward the northern and south- 

 ern limits of this zone the Foraininifera diminish, while Fui- 

 diolaria remain and Diatomaccce increase in proportion, so 

 that, in the circumpolar areas north and south of G0° in each 

 hemisphere, tlie surface-organisms are chiefly such as have 

 silicious skeletons. In accordance with this condition of the 

 surface-life, the ooze covering the sea-bottom in these regions 

 is no longer calcareous but silicious, being composed of the 

 cases of Diatoms and the skeletons of Fadiolaria often 

 largely mixed with ice, drifted mud, stones, gravel, and bowl- 

 ders. 



If we suppose the globe to be uniformly covered with an 

 ocean 1,000 fathoms deep, the solid land forming its bottom 

 would be out of reach of rain, waves, and other agents of 

 degradation, and no sedimentary deposits would be formed. 

 But if Foraminifera and Diatoms, following the same huvs 

 of distribution as at present obtain, were introduced into 

 this ocean, the fine rain of their silicious and calcareous hard 

 parts would commence, and a circumpolar cap of silicious 

 deposit would begin to make its appearance in the north and 



