INTRODUCTION 49 



differs from the species in a less degree than two aUied 

 species. It may also be differentiated by the fact 

 that a true species usually breeds true to type from 

 seed under cultivation, whilst a variety may or may 

 not do so. It may, however, be propagated vegeta- 

 tively and remain constant or not. If there be some 

 main character which distinguishes a set of indivi- 

 duals from one another (but one presumably belonging 

 to the same species), it may be regarded as a sub- 

 specieSf when the character by which it is recognised 

 is found to remain permanent through a series of 

 generations. By some the term race is used in the 

 same sense. When the character is less permanent 

 the individual may be regarded as a form, and when 

 it reverts to the type such a form exhibits reversion 

 or atavism. Sports are impermanent forms, and may 

 be abnormal variations due to injury or some other 

 cause. Double flowers are of such a type, though 

 they may be more permanent than such monstrous 

 characters as the peloria state of the Snapdragon. 



All these groups of individuals, species, varieties, 

 forms, etc., may be arranged together in a genus, 

 which includes a number of species (or sometimes 

 only one) which differ singly in some one character 

 (or more than one) from others. The genus, how- 

 ever, combines in its range plants which are all 

 connected by some character or characters that 

 distinguish them from another genus, and these are 

 convenient groupings which help to show by a further 

 stage the history of the group or the individual, or 



VOL. III. 4 



