12 Descent 



In order to meet this difficulty we must recog- 

 nize two sorts of species. The systematic spe- 

 cies are the practical units of the systematists 

 and florists, and all friends of wild nature 

 should do their utmost to preserve them as 

 Linnaeus has proposed them. These units how- 

 ever, are not really existing entities; they 

 have as little claim to be regarded as such 

 as genera and families. The real units are 

 the elementary species; their limits often ap- 

 parently overlap and can only in rare cases be 

 determined on the sole ground of field-obser- 

 vations. Pedigree-culture is the method re- 

 quired and any form which remains constant 

 and distinct from its allies in the garden is to 

 be considered as an elementary species. 



In the following lectures we shall con- 

 sider this point at length, to show the compound 

 nature of systematic species in wild and in culti- 

 vated plants. In both cases, the principle is 

 becoming of great importance, and many pa- 

 pers published recently indicate its almost uni- 

 versal acceptation. 



Among the systematic subdivisions of species, 

 not all have the same claim to the title of ele- 

 mentary species. In the first place the cases 

 in which the differences may occur between 

 parts of the same individual are to be excluded. 

 Dividing an alpine plant into two halves and 



