XXVI INTRODUCTION, 



The new valves formed during the process of self-division or- 

 dinarily appear exact counterparts in form and size of the valves 

 of the original frustule ; but a careful examination of the pro- 

 cess in the filamentous species has led me to conclude that a 

 slight enlargement occasionally takes place in the new valves, 

 thus causing a widening of the filament. 



This increase is however so small, that in a filament of many 

 hundi'ed frustules, the enlargement is scarcely appreciable. 

 The rapid attenuation represented by some authors in the fila- 

 ments of the Fragilanm must therefore be attributed to the 

 deceptive appearance presented by a compressed band when 

 slightly twisted, the semblance of attenuation being thus given 

 to the portions which are presented in an oblique chrectiou to 

 the eye of the observer. 



The increase in the new valves, although slight, will however 

 sufficiently account for the varying breadth of the bands in the 

 filamentous species, and the diversity of size in the frustules of 

 the free forms, without obliging us to suppose that a growth or 

 aggregation takes place in the siliceous valve when once formed. 

 Starting from a single frustule, it will be at once apparent, that 

 if its valves remain unaltered in size, while the cell-membrane 

 experiences repeated self-division, we shall have two frustules 

 constantly retaining their original dimensions, fom' slightly 

 increased, eight somewhat larger, and so on, in a geometrical 

 ratio, which will soon present us with an innumerable multitude 

 containing individuals in every stage, but in which the larger 

 sizes preponderate over the smaller ; and such are the ciixum- 

 stances ordinarily found to attend the presence of large numbers 

 of these organisms. 



It is hardly necessary to remind the student acquainted with 

 the general laws of the vegetable kingdom, that this multipli- 

 cation of the Diatomaceous frustule is not a true reproduction 

 of the species, but merely a growth or extension of the original 

 frustule with which we may suppose self-division to originate ; 

 and as the myriads of cells which constitute the entirety of the 

 largest sea- weed, are regarded in theh^ aggregation as forming 

 but one plant, so may we consider the thousand millions into 

 which a single month's growth has multiphed the original frus- 



