RAPIDITY OF INCREASE. 13 



or " frustules," as they are also termed, are entirely disconnected 

 from each other, but in others they are. united together in con- 

 siderable masses in a sort of gelatinous envelope, and in others, 

 again, they are variously united into composite structures, which 

 sometimes present an appearance of great elegance and beauty. 

 In the more simple of these associated tribes, the frustules are 

 merely held together in long filaments, or zigzag chains ; in others, 

 they are arranged, side by side, in flat bauds or beautiful screw- 

 like coils ; while in some very elegant forms they are mounted on 

 slender foot-stalks, and have the appearance of clusters of tiny 

 fans, every ray of each separate fan consisting of a perfect Diatom 

 enclosed in its silicious shield. 



Like their allies the Desmidiaceee, the Diatomacece increase 

 both by self-division and by conjugation, and so rapidly does their 

 multiplication proceed, that one of the most cautious of micro- 

 scopical observers has calculated that, in the course of one month, 

 the progeny of a single Diatom would amount to the amazing 

 number of one thousand millions ! This extraordinary rapidity 

 of increase will readily account for the sudden appearance of vast 

 numbers of these organisms in localities where, only a short time 

 before, they were either not to be found at all, or occurred but 

 sparingly ; it will also help us to account for those vast accumu- 

 lations of their silicious shields which are being formed in various 

 parts of the ocean at the present day, and the still more extensive 

 deposits of similar exuvias which originated in remote geological 

 epochs. It has already been mentioned that the shields of the 

 Diatomaceas are indestructible. Unlike, therefore, to multitudes 

 of higher organisms, which live their day and then disappear, 

 leaving no trace of their existence behind them, every one of 

 these lowly beings deposits in the water it inhabits an imperish- 

 able memento of its brief life, and not unfrequently contributes to 

 the formation of rocky strata, which, in the remote future, may 

 give a distinctive character to countries yet unborn. 



In some places the accumulations of these remains are exerting 

 an important influence in blocking up harbours, and diminishing 

 the depth of channels; and, according to the observations of 

 Professor Bailey of New York, various portions of the bottom ol' 

 the North Pacific Ocean are being covered up by deposits of a 

 similar character, in which the two valves of the shield are 

 frequently found still united, and containing remains of the 



