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after the death of the individuals to which they refer, would be 

 useless for the purpose of identifying living plants and would, 

 therefore, be of little value to us or our successors. It has, 

 however, long been recognised that although the immediate 

 offspring of any individual usually differ slightly from their 

 parents and each other, they, on the whole, invariably resemble 

 their parents and each other very closely, and this undoubted 

 fact that, within certain limits, all organisms breed true, 

 affords the only basis for a natural history classification which 

 shall be of practical value. By discovering within what 

 limits each different kind of plant breeds true, i.e. *by determin- 

 ing which characters are always transmitted truly to its 

 immediate offspring, we are able to obtain a unit which, so far 

 as we can see, is permanent, the marks which distinguish the 

 individuals belonging to this unit from all other plants being 

 always transmitted unchanged from parents to offspring 

 through successive generations. Such a unit can conse- 

 quently be recognised and studied by our' successors, while, by 

 only giving a separate name to each such unit instead of to 

 each individual, the number of names which will be re- 

 quired is enormously reduced. Such considerations have led to 

 the selection of the so-called species as the unit of classification. 

 It will be seen also that the morphological characters which 

 are most important in classification are those which are 

 always transmitted unchanged from parents to their offspring 

 and which therefore indicate genetic relationship. 



Species, ( 108 With these preliminary remarks 



Variety, * ' tne following definitions are now given : 



Genus. DEFINITION 1. A SPECIES is the smallest group of plants 



existing wild in nature which can be readily distin- 

 guished from all other groups owing to the fact that the 

 individuals composing it all possess in common certain 

 well marked characters (= specific characters) by 

 which they can be distinguished from all other plants. 

 The individuals also which compose the species are, 

 when developed normally in a state of nature, always 

 able to transmit their specific characters unchanged to 

 the majority of their immediate offspring. 



DEFINITION 2. A SUB-SPECIES is a group essentially 

 similar to a species but subordinate to it. The 

 differences separating any two individuals belonging to 

 different sub-species not being so great as those which 

 separate individuals belonging to different species. 



