! various important subjects in Mineralogy . 27 
ly in their isomorphous constituents, must necessarily be con- 
sidered as belonging to one and the same species, because these 
isomorphous substances are often but partially exchanged, and 
not in their whole quantity, so that a composition of both in va- 
rious proportions, often takes the place of the one or the other. 
There is thus produced a kind of chemical transition, which ren- 
ders it necessary to collect all those varieties within one and 
the same species, if we wish to avoid what would result from 
assuming too many of them, the entire destruction of the idea 
of the chemical species. 
The species is the lowest among the systematic ideas in Natu- 
ral History : For, if we proceed from the identical individuals, 
and unite them with whatever may be done so according to the 
series of characters, among which those of the regular forms are 
the most important, because they impart security to the em- 
ployment of the rest of the series ; then we immediately arrive 
at the idea of the assemblage of those homogeneous individuals 
which produce the species of Natural History. A farther distri- 
bution of the varieties into subspecies or hinds is reprehensible, 
because it is without the slightest advantage in a scientific point 
of view ; impedes the easy survey of the species ; and renders 
the nomenclature difficult or inconsistent. The species in Natu- 
ral History, although the lowest, is therefore the foundation of' 
all the higher ideas , in the same way as it is the formation of 
all those sciences different from Mineralogy, which refer to the 
productions of the mineral kingdom. 
After the idea of the species, that of the Genus comes next to 
be considered. If, in Natural History, we have in view to pro- 
ceed with consistency, the determination of this idea must be 
entirely dependent upon natural-historical principles. This being 
the case, it is evident what opinion we ought to form of such sys- 
tems as have their species determined according to principles of 
Natural History, and their genus according to those of Chemistry. 
It would even seem that this want of consistency has been long ago 
understood, but that the difficulties attending its removal have 
appeared too formidable to be overcome. Yet this want of con- 
sistency is the greatest evil in every science. If it were impos- 
sible to find a principle, according to which the determination of 
