INTRODUCTORY. 235 



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. 61). 



Considered with reference to plants now actually living, the 

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

 certain of them, as the Equisetinse, include few forms, and those 

 for the most part very closely allied, others, as the Dicotyledones 

 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 Equisetinae or the Lycopodinae, are few, they are but the sur- 

 viving 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 

 the genus, and the second that of the species. Thus, for instance, 

 the greater Plantain, Plantago major, and the Ribwort, Plantago 

 lanccolata, are two species of the genus Plantago. Since in early 

 times the same plants were often described under different names, 

 and as different plants were often designated by the same name, it 



