GUANO. 165 



quantity. Thus, the dung of wild ocean birds yields 

 a larger revenue to the Peruvian exchequer than all 

 the silver mines of Cerro de Pasco ; and its transport 

 occupies greater fleets than ever Spain possessed at 

 the proudest height of her maritime ascendancy. 



Now Diatomacece form a very considerable per- 

 centage of the entire bulk of this substance, the value 

 of which is augmented in proportion to the abundance 

 of these microscopic organisms. Great masses may 

 often be found wholly composed of the aggregated 

 frustules of Diatoms. How are these procured in such 

 vast supply ? It has been by some supposed that the 

 birds, or that fishes on which they subsist, feed directly 

 on them. But this is manifestly untrue, as Dr Wal- 

 lich has shown, since, with one or two rare exceptions, 

 Diatomaceous frustules are sufficiently large to be 

 appreciable by any bird's eye. Nor could any verte- 

 brate animal we are acquainted with, by any possi- 

 bility, gather together, within a reasonable period, a 

 sufficient supply of such infinitesimally minute nutri- 

 ment as these organisms afford, even supposing the 

 optical difficulty to be overcome. Nor could any 

 prehensile or masticatory apparatus deal with it, if 

 taken into the mouth : it must be swallowed en masse. 



But the intervention of swarming hosts of inver- 

 tebrate animals solves the difficulty. It is well known 

 that the vast tribes of bivalve Mollusca are supported 

 almost wholly on these and similar entities ; which are 

 taken, without any craft, or violence, or pursuit, or 

 even selection, by the mere action of ciliary currents 



