24 Professor Mohs’s General Reflections on 
these combinations, every form is excluded which does not be- 
long to the series. We are then entitled to ask why, among the 
numberless combinations in which the individuals of rhombohe- 
dral lime-haloide (calcareous spar) appear, there never occurs a 
form belonging to the prismatic system, nay, not even a rhom- 
bohedral form that could not be derived, according to the well- 
known geometrical processes, from the same fundamental form ? 
Why, inversely, there never appear combined with the rest of 
the forms of the prismatic lime haloide (arragonite), any rhom- 
bohedrons, or isosceles and scalene six-sided pyramids, regular 
six-sided prisms ? &c. The answer to this question is, Because 
the rhombohedral and the prismatic lime-haloide are two different 
species, and because nature combines the various simple forms 
only within the limits of one and the same species, to the entire 
exclusion of all the rest. So long, therefore, as there is no ex- 
ception to this rule, which is established upon experience, and 
which can be contradicted by experience alone, we possess in the 
fact of their being incompatible, an incontrovertible criterion 
of the differences between natural-historical species. Hence 
the inferences to which allusion has been made above, appear 
groundless. 
The object of terminology having been thus determined, we 
have now to develope those general ideas and representations, 
which in particular might be called natural-historical ones, and 
of which those that regard the species are the most important. 
They are produced by considering the natural-historical pro- 
perties, not by themselves, as in terminology, but in connection 
with each other, and by considering the natural productions 
themselves which possess them. Though these ideas have been 
already developed, and are generally known, (circumstances which 
render it sufficient to give a brief account of them in this place), 
yet it will be useful not to pass them over in silence, but to ex- 
hibit them in their connections, since there are some among 
them which apply not only to the species, but also to the genus, 
the order, &c. 
The first is the idea of Species , which indicates that the spe- 
cies is the assemblage of homogeneous individuals, that is to 
say, those whose natural-historical properties which may be ob- 
served while the mineral continues to exist, are either absolutely 
