48 THE STORY OF PLANT LIFE 



individuals which are essentially alike in more 

 characters than one, or differ only in a small degree, 

 and may be regarded as derived by evolution (or the 

 derivation of later and more highly organised plants 

 from earlier or less specialised types) from a common 

 ancestor. The exact definition of a species in the 

 case of plants is not easy to frame, because plants 

 are more plastic or more liable to vary than animals, 

 though even in the latter case, again, definitions as to 

 specific differences or identities are not easy to adopt. 

 If it is remembered, hov^ever, that terms are relative 

 only, and that actual boundaries between two forms 

 are liable to vary, the difficulties of nomenclature as 

 to fixity of specific forms is somewhat diminished. 

 We recognize that nomenclature is elastic, and is, 

 therefore, progressive. The present state of our 

 knowledge is capable of readjustment or improve- 

 ment, and no boundaries are rigid. 



We may thus admit that an original species may 

 be connected with another which is essentially 

 different in some more or less important character 

 by the existence of other individuals, or grades of 

 individuals, which we may call varieties. The end- 

 less types of individuals which link up two allied 

 species, or groups of individuals, may be connected 

 by such intermediates or varieties ; and the denoting 

 of certain striking types by such terms as variety may 

 help us to fix more accurately or more conveniently 

 the degrees of relationship or difference between the 

 extreme types. The variety, as a rule, therefore 



