INTRODUCTION. XXVll 



tule, as belonging to the individuality of the jfirst organism in 

 the Diatomaceous chain of being. 



With the free species of the Diatomacese the analogy may 

 seem a forced one ; but its correctness is at once apparent, if 

 we ascend to more complex combinations in the filamentous 

 and frondose genera, no very obvious difference existing be- 

 tween the union of cells in Fragilaria and those of many spe- 

 cies of the ConfervcB ; or their aggregation in Sckizonema, and 

 various forms of the larger Algae. The most important distinction 

 between the combination of cells in the two cases, is the greater 

 readiness with which the connection is dissolved in the Diato- 

 macea3 and the retention of independent life in each separated 

 cell; but as we descend in the scale to the lower Algae, and 

 approach the UlvacefS or Palmellacea, even this difference dis- 

 appears, and the smallest portion of the frond, even a simple 

 cell, in these forms enjoys the same independent vitality and 

 power of increase as the Diatomaceous frustule. 



The increase by self-division in the Diatomacese is therefore 

 not a renewal, but a mere extension of individual life ; and the 

 process has been justly defined by Mr. Thwaites, in the analo- 

 gous case of the Palmellacea, as an act of Gemmation rather 

 than Reproduction. 



" If we duly consider this fact," remarks this acute and zeal- 

 ous student of nature, " how much does it exalt the lower tribes 

 of plants in our estimation ! since we may contemplate an indi- 

 vidual plant of them not as the single phyton — not as the single 

 frond — not as the single cell — but as the aggregate, it may be, 

 of thousands of these ; — view it occupying as much space, and 

 exercising as great an influence in the oeconomy of natm^e, as 

 the largest forest tree." (Annals of Natural History, 2nd Series, 

 vol. ii. p. 315.) 



Section VI. 

 Terminology and Classification. 



The circumstances attendant upon the self-division of the 

 Diatom furnish means for fixing the terminology apphed to the 



