PART IV.

T A X O N O M Y.

Taxonomy is that branch of Science which investigates the prin-ciples of System. It embraces the two dependent subjects. Classi-fication and Nomenclature.

CHAPTER I.

CLASSIFICATION.

IDENTITY, AND NON-IDENTITY OF SPECIES.

107. Abstractly considered, those individuals are conspecificthat possess all the invariable qualities of a particular species. It istherefore important to understand the relative value of crystallo-graphic, physical, and chemical characters, in the determination ofidentity.

a. Importance of Crystallization in Determining Identity.

The characters of minerals depending on crystallization, hold thefirst rank. The present elevation and perfection of the science ofMineralogy , has mostly arisen from the developments of the prin-ciples of crystallography. Indeed, the only characters in whichthe mineral species are invariably constant, are the form andstructure of their crystals. The following canon, which was firstintroduced by the Abbe Haiiy, is therefore fundamental in the sci-ence, and should take the precedence of all other laws or princi-ples : Similarity of crystallization proves an identity of species,and dissimilarity a non-identity. When a mineral presents per-fect and distinct crystals, no other character is required for its de-termination. By similarity of crystallization is understood, notmerely an identity in the system of crystallization, but also, in an-alogous interfacial angles, and in cleavage. Occasionally, there is aslight variation in the angles, and in the cleavage of some of the