218 PART III. THE CLASSIFICATION OF PLANTS. 



Phanerogamia may be contrasted, as seed-bearing plants, with 

 the three groups (Thallophyta, Bryophyta, Pteridophyta) of plants 

 which do not bear seeds, and which are collectively termed Cryp- 

 togamia. 



furthermore, the Thallophyta are characterised by the fact that 

 the female organ is never an archegonium, whereas in the other 

 three groups it is never anything else than an archegonium, though 

 it may present variations of form and structure (see p. 84) : the 

 Bryophyta, Pteridophyta, and Phanerogamia may, on this account, 

 be collectively designated Archegoniata. 



The above-mentioned Classes are of very unequal extent ; for 

 while certain of them, as the Equisetinae, include few forms and 

 those for the most part very closely allied, others, as the Dicotyle- 

 dones and the Fungi, include an enormous number of very different 

 forms. These discrepancies arise from the very nature of the 

 natural system, for a great diversity does not necessarily display 

 itself within the limits of a single Class; and it must not be for- 

 gotten that when the living representatives of a Class, for instance 

 the Equisetina3 or the LycopodinaB, are few, they are but the 

 surviving remnant of once various and numerous forms which 

 have become in great measure extinct. 



Those Classes which include a sufficiently large number of forms 

 are subdivided into subordinate divisions, as (1) Sub-classes, (2) 

 Series, (3) Cohorts, (4) Orders, and these again, if necessary, into 

 Sub-orders, etc. ; but these names are applied in the most arbitrary 

 manner to the different sub-divisions. The two narrowest system- 

 atic conceptions, viz., Genus and Species, are used to indicate an 

 individual plant. Under the term Species are included all in- 

 dividuals which possess in common such a number of constant 

 characters that they may be considered to be descended from a 

 common ancestral form. New peculiarities may arise in the 

 course of multiplication : the individuals characterised by these 

 new peculiarities are regarded in classification as varieties of the 

 species. When several species resemble each other so distinctly 

 that their general characters indicate a relationship, they are 

 grouped together in a Genus. The limits of genera are conse- 

 quently by no means fixed, but vary according to the views of 

 individual botanists. In the larger genera the species are grouped 

 into Sub-genera. 



The scientific name of every plant consists on the plan intro- 

 duced by Linnaeus of two words, the first indicating the name of 



