270 THE BEAN FAMILY. 



The relative values of the characters which distinguish what we call 

 varieties, species, and genera can be illustrated by comparing, for 

 example, the resemblances and differences existing between the varie- 

 ties of Sweet Pea, Garden Pea, Broad Bean ; those existing between 

 the different species of the genera Laihyrua, Vicia, Trifolium, etc. ; 

 and those between the different genera of Leguminosae. 



The Order Leguminosae is united with several others to form the 

 Cohort Resales, which includes the Orders Rosaceae, Saxifragaceae, 

 Crassulaceae, and a few more. If only the native British genera of 

 Leguminosae are taken into account, it is hard to see why this order 

 is grouped with those just named, but the Mimosa-tribe of Legumi- 

 nosae has flowers which differ very little in structure from those of 

 certain Rosaceae. 



As you proceed with the work of classifying plants, you 

 will realise that the boundaries between varieties and species, 

 between species and genera, and so on, cannot be precisely 

 and sharply defined ; the same applies even to the boundary 

 between the lowest plants and animals. This is, of course, 

 just what one might expect from what has been said about 

 variation and evolution. Hence an absolutely natural system 

 of classification can never be realised. The nearest approach 

 to such a classification of Angiosperms is that of Engler and 

 Prantl, adopted in this book, which should be compared with 

 the Bentham-Hooker arrangement given in British Floras. 



The relations between a variety, a species, a genus, and an order 

 may be shown as below : 



Variety 



, Variety 



Genus ^ 



Variety 



Variety 

 Order/ 



Variety 



GenU8 Variety 



^ T . 

 Variety 



Of course, there are many species which, though variable (as all 

 species are), have no distinct varieties that is, they show continuous 

 but not discontinuous variation. A species may have many or few varie- 

 ties, a genus many or few species, and an order many or few genera. 

 In fact some genera have only one species that is, a species may be 



