Wallich^ on the Diatom-valve. 
131 
ible nature of wliich_, coupled with their extreme trans- 
parency and the already understood chemical relations of 
their siliceous element, renders them so well fitted for the 
inquiry. 
It is admitted on all sides that the rules by which species 
have heretofore been established demand material modifica- 
tion. Amongst the Diatomaceae, aline, a dot, a minute spine, 
or a variation in size or outline, amounting, relatively speak- 
ing, to nothing more than we see occurring amongst indi- 
viduals of the same species of every animal and plant in 
nature, has been accepted, by some observers as of sufficient 
moment to constitute a specific character. Almost every 
microscopist has fallen more or less into this error. But it is 
by no means an error confined to one class of observers ; for 
entomologists, botanists, and conchologists are in exactly 
the same predicament. So that our knowledge of the minute 
Fauna of the globe, after all, is but in the same state as that 
of other branches of natural history, making due allowance 
for the multiplied difficulties by which microscopic investi- 
gation is specially beset. 
Now, in the case of the Diatomacese, the leading generic 
characters are taken from the configuration of the siliceous 
valve. The sooner, therefore, that we gain such an insight 
into the law which regulates the deposit of the silex, as shall 
enable us to judge how far the diff'erences of configuration 
may be dependent on chemical or molecular forces, and how 
far on the inherent property of the organisms themselves, the 
sooner shall we have it in our power to establish a natural 
classification, and to simplify, by rendering determinate, the 
methods of investigation. 
We have before us the phenomenon of a mineral, elimi- 
nated or secreted by what we are pleased to denominate a 
simple cell, in a state of the utmost purity, and assuming 
definite forms, which may be said to hold an intermediate 
position between crystallization and simple molecular aggre- 
gation. We know the fact, but as yet we know absolutely 
nothing of the causes producing it. Surely then the investi- 
gation of such a problem is worthy of the best efibrts of the 
microscopist ; and from this point of view, even the " dot^^ 
may exercise a world of significance. 
But whilst it is necessary that we should possess a clear 
idea of the structure of the siliceous wall of the diatom, it is 
by no means essential that we should have at our finger ends 
the precise nature of the surface in the more minute and 
subtle forms, or indeed in any save those that are typical 
and most easily revealed. If analogy means anything at all, 
