42 THE STORY OF THE EARTH AND MAN. 



cal fact in such wise that phosphate of lime is a most 

 appropriate and suitable material for its teeth and 

 bones. Now, in the case of the lower animals of the 

 sea, their food, not being of the nature of the richer 

 land plants, but consisting mainly of minute algae 

 and of animals which prey on these, furnishes, not 

 phosphate of lime, but carbonate. An exception to 

 this occurs in the case of certain animals of low grade, 

 sponges, etc., which, feeding on minute plants with 

 siliceous cell-walls, assimilate the flinty matter and 

 form a siliceous skeleton. But this is an exception 

 of downward tendency, in which these animals ap- 

 proach to plants of low grade. The exception in the 

 case of Lingulaa is in the other direction. It gives 

 to these humble creatures the same material for their 

 hard parts which is usually restricted to animals of 

 much higher rank. The purpose of this arrangement, 

 whether in relation to the cause of the deviation from 

 the ordinary rule or its utility to the animal itself, 

 remains unknown. It has, however, been ascertained 

 by Dr. Hunt, who first observed the fact in the case 

 of the Primordial Lingulae, that their modern suc- 

 cessors coincide with them, and differ from their 

 contemporaries among the mollusks in the same par- 

 ticular. This may seem a trifling matter, but it 

 shows in this early period the origination of the dif- 

 ference still existing in the materials of which animals 

 construct their skeletons, and also the wonderful per- 

 sistence of the Lingulaa, through all the geological 

 nges, in the material of their shells. This is the more 



