BOTANICAL CLASSIFICATION. 331 



662. The fixity of species under this view is not absolute and 

 universal, but relative. Not, however, that specific changes are 

 necessitated in virtue of any fixed or all-controlling natural law. 

 Some of the lowest forms have existed essentially unchanged 

 through immense geological periods down to the present time ; 

 some species even of trees are apparently unchanged in the lapse 

 of time and change of conditions between the later tertiary period 

 and our own day, during which most others have undergone 

 specific modification. Such modifications are too slow to effect 

 in any wise the stability and practical application of botanical 

 classification. 



SECTION II. BOTANICAL CLASSIFICATION. 



663. Natural and Artificial Classifications may be distinguished. 

 A natural classification in botany aims to arrange all known plants 

 into groups in a series of grades according to their resemblances, 

 and their degrees of resemblance, in all respects, so that each 

 species, genus, tribe, order, &c., shall stand next to those which 

 it most resembles in all respects, or rather in the whole plan of 

 structure. For two plants may be very much alike in external 

 appearance, yet very different in their principal structure. Arti- 

 ficial classifications single out one or more points of resemblance 

 or difference and arrange by those, without reference to other 

 considerations, convenience and facility being the controlling 

 principles. The alphabetical arrangement of words in a dic- 

 tionary, and the sexual system in botany by Linnaeus (or rather 

 a part of it) , in which plants are arranged in classes upon the 

 number of their stamens, and in orders upon the number of 

 pistils, are examples of artificial classification. The arrange- 

 ment of the words of a language under their roots, and with the 

 derivative under the more primitive forms, would answer to a 

 natural classification. 



in the life of the plant, and which therefore undergo modification under 

 changing conditions. Unessential structures accordingly are left unaltered 

 or are only incidentally modified. And so these biologically unessential 

 points of structure, persisting through all adaptive changes, are the clews 

 to relationship. Thus, Rubiaceae are known by insignificant stipules, Ano- 

 naceae by ruminated albumen, Rhamnaceae by a valvate calyx and stamens 

 before the petals, &c. Paradoxical as it may seem, it is not although, but 

 because, they are of small biological importance that they are of high clas- 

 sificatory (i. e. of genealogical) value. 



On considerations like these, characters are divided into adaptive or bio- 

 logical on the one hand, and genealogical or genetic on the other. The saga- 

 cious naturalist seizes upon the latter for orders and the like ; while the 

 former are prominent in genera, &c. 



