Translated by Philip H. Nicklin. i 113 
one exclusively, is entirely wanting, and will perhaps always be 
so, if I am right in classing among impossibilities, the only means 
of bringing about such assent. 
What is to be done, then, in the midst of all these difficulties ? 
How shall we escape the horns of this dilemma; reform is twant- 
ed, but permanent reform is at this time impossible ; nevertheless, 
on the other hand, i is absolutely necessary to classify the a 
already known, and those which are discovered ever day. There 
is nothing but provisional arrangement, nothing but system, (and 
system is artificial method,) which can rid us of this embarrass- 
ment. Let us, then, consult theory, experience, analogy, that 
we may labor to come at the truth. 
Theory says, that there are essential differences between genus 
and genus, between species and species. ji 
Experience tells us, that dissimilar animal are sometimes found 
in similar shells, and animals almost identical in shells, apparently 
very different. e 
Analogy proposes, under these premises, to draw inductions 
from the mass of discoveries already made. — 
Let us take then this immense group, such as MM. de Blain- 
ville and Deshayes understood it a few years ago; this group, 
Whose animals they then believed to be generically identical ; let 
us begin by lopping off, finally, with M. Deshayes, the ridin, 
with M. D’Orbigny the Castalia, of which the shell alone did 
not permit a rational distinction. : Then let us reduce, with Mr: 
Lea, all the other genera of the Naiades,* to the simple im- 
portance of subgenera or sections, waiting always to withdraw 
from these, all those species whose anatomical characters when ~ 
known, shall be found to differ from those of the studied type, in 
like manner as the Iridine and Castalia, and we shall have a 
genus which will possess only a supposed unity in its essential 
characters, but which on that very account will be conditionally, 
and therefore theoretically, exact. — 
Arriving at species, we would proclaim as irrevoca ) 
mous, those which offered no specific anatomical different 8, 8 
as the form of the ovaries studied by Mr. Lea in the Unio irro- 
ratus, ochraceus, cariosus, retusus, in the Anodonta undulata and - 
al 
*T say nothing here of the genus Mycetopoda of M. d’Orbigny, which I have 
Not at present an opportunity of studying. Mr. Lea does not admit LL any more 
than that of Castalia. : 
Vol, xu1, No. 1.—April-June, 1841. vas 
