ANIMAL NATURE OF DIATOME^J. 491 



all, the differences may be produced only by a slight 

 modification in form, and principally in the dimensions, 

 of the organic elements. 



It remains to be decided what this structure is. The 

 shield of the Biddulphieae seems to be a very simple 

 cellular tissue. It is true, sufficient observations on its 

 origin, formation, and successive growth, are wanting ; 

 but although we have no direct observations (which 

 may deceive), we ought necessarily to admit it to 

 be such. It will then be the same as that of other 

 Diatomese, which often Appears to be a very simple, 

 uniform, and continuous vitreous lamina, but this appear- 

 ance, perhaps, arises from the minuteness of the cells of 

 which it is constituted, or from their complete obstruction 

 and perfect continuity. 



Whatever be the structure of the Areolatae, it can 

 never be regarded as belonging to the walls of a cell. 

 Indeed, on the supposition of the vegetable nature, and of 

 the inferior rank that the Diatornese would occupy in 

 the class Alga3, we must assert for these, as for all the 

 others, the same condition of a simple cell. For in a 

 system where the distinction of monogonimic, poly- 

 gonimic, and ccelogonimic cells is adduced as of high 

 taxonomic value, a character of so much higher import- 

 ance cannot be lightly neglected. 



The study of microscopic beings presents much greater 

 difficulties than that of the microscopic parts of larger 

 organisms. The ideas that we habitually form to our- 

 selves of the great and the small, are relative to the 

 capacity of our senses, and he who sharpens those senses 

 the most by the aid of physical instruments, most extends 

 the relations of those ideas, to distant confines. In this 

 way we come to persuade ourselves, that in nature there 

 exists no absolutely great nor absolutely small. The 

 minutest organisms are not, therefore, the most sim- 

 ple in construction, and their organisation frequently 

 baffles all means of observation. It follows, from these 

 reflections, that in the study of beings of a given family 



