490 ANIMAL NATURE OP DIATOME^. 



of Diatoraeae, considered as organised beings, we find 

 ourselves of necessity, reduced to a dilemma. Either the 

 shield of the Areolatae has a structure entirely different 

 from that of the other Diatomese, or the shield of the 

 latter has a compound structure, which escapes the eye, 

 though armed with the highest magnifying powers, 

 owing to the tenuity, and minuteness of its elementary 

 parts. Perhaps some one may attempt to elude the 

 question, by admitting that this shield can be no other 

 than a product of secretion. But, besides that, the 

 products of secretion are living beings, organised and 

 susceptible of their own ulterior organic modifications, 

 as we have previously demonstrated : in either mode it is 

 necessary to admit a particular organisation in the se- 

 creting organ, because its products assume a particular 

 configuration and texture. If, therefore, the vital and 

 organic power be understood as limited solely to the soft 

 substance, and extended afterwards to the solid, it always 

 comes to be the same thing — the peculiarity of organisa- 

 tion. The difference is reduced to the soft or solid 

 constitution; nor does this isolated condition prove, in 

 my opinion, the mineral condition of a tissue which does 

 not present any character either of crystallization or 

 inorganic deposition. 



Of the two discordant opinions, I believe we ought 

 to admit that which supposes by analogy the usual 

 elementary structure to prevail in the shields of all 

 Diatomese. And, in truth, the presence of striae, points, 

 depressions, elevations, manifests, in many even of those 

 not areolated, a structure neither uniform nor continuous. 

 We find, too, that independently of this condition, many 

 degrees of correlation connect the genera of the last tribe 

 to some of those of the two preceding. The Odontellce 

 are rightly placed near the Biddulphiee and Isthmice, 

 though they do not possess an areolated shield. And if 

 a decided organic affinity be undeniable in the entire 

 organisation of such beings, it is reasonable to suppose, 

 that even in the structure of an organ common to them 



