EEPORT ON THE DIATOMACE.E. 1 1 



the same conclusion. Here Diatoms of different species do occur, but they are relatively- 

 scarce — a circumstance which may be explained by the fact that no large rivers flow into 

 this area to compensate for the enormous evaporation to which it is subjected on account 

 of its vicinity to the equator, so that its waters are more dense and more heavily charged 

 with saline constituents. 



If this explanation be correct, it follows that the warmest, and, by their great 

 evaporation, the most saline parts of the ocean, serve as all but insuperable barriers 

 to the mixture of different floras, so that each may retain almost indefinitely its own 

 special characteristics. 



Silica, of which the walls of Diatoms are composed, exists in all kinds of water, but 

 generally in so small quantities that its proportion cannot be determined. Hence 

 Professor Bischoff has lately asserted that siliceous Infusoria — meaning thereby Diatoms — 

 must take into their interior, in order to form their valves, the same quantity of water, in 

 proportion to their mass, as would be swallowed by a man who drank a cubic foot per 

 second. 



If water were absolutely devoid of silica, Diatoms could not exist, but this and other 

 important substances are being continually carried into the sea by rivers. The calcareous 

 material present in the ocean goes to form the tests of Foraminifera and Crustacea and the 

 coral formations of tropical zones, whfle the silica is removed by Polycistinse, Radiolaria, 

 certain Foraminifera, Spongidse, and Diatoms. 



The secretion of lime takes place, according to Mr. Murray, most actively in the 

 warmest and saltest seas where solar evaporation is most rapid, and it is a fact no less 

 striking that the greatest abundance of Diatoms is found in localities where the water is 

 of relatively low specific gravity and the temperature in some instances likewise low. 



THE BATHYMETRICAL LIMIT OF DIATOMS. 



Problems of distribution are, from the very nature of the conditions, much more 

 difficult to solve in the case of marine than of terrestrial organisms, and with regard to 

 the Diatomaceae it has hitherto been impossible to define the greatest depths at which they 

 are capable of surviving in the various ocean basins. A method has been already referred 

 to by the application of which it may be proved whether they do vegetate at any given 

 depth (pp. 2, 3), but in addition to this, careful records should be made of the occurrence 

 of living frustules on fragments of corals, shells, &c, which are brought up from a known 

 depth, as a check is thereby placed on the experimental working out of this most 

 important question. 



From a knowledge of their physiological functions of decomposing carbonic acid and 

 evolving oxygen under the influence of sunlight, it may be inferred that Diatoms cannot 

 live where light is unable to penetrate. "With this theoretical conclusion daily experience 



