THE BEAN FAMILY. 269 



but other characters (adhesion, form of receptacle, symmetry, 

 number of parts in a whorl, placentation, etc.) are also used, 

 as are also the characters of the fruit and seed (e.g. number 

 of cotyledons, form of embryo, presence or absence of 

 endosperm). 



297. Variety, Species, Genus. The object of classifica- 

 tion is to arrange plants in a scheme which shall as nearly as 

 possible express their actual affinities or relationships by 

 Descent, or Evolution, from common ancestors. A really 

 natural scheme can never be more than an ideal to be aimed 

 at, and any scheme adopted is only an expression of our views 

 concerning these "blood relationships," but as knowledge 

 grows by further research the scheme will become more and 

 more a true reflection of the affinities of plants. 



There is always a certain amount of variation among the 

 offspring of plants, no two individuals being exactly alike. 

 In most cases variation is continuous, or fluctuating, i.e. 

 the varying forms are connected by a continuous series of 

 intermediate forms. In some cases, however, it is discon- 

 tinuous, i.e. we find on examining a large number of indi- 

 viduals that they group themselves in two or more sets, races, 

 or varieties, which are hardly connected, if at all, by inter- 

 mediate forms. Many examples of both kinds of variations 

 are found in cultivated plants. Varieties differ from each 

 other in relatively small and variable characters, often only 

 in the vegetative organs or in floral or seed characters, of 

 slight importance. 



Species are groups which differ in more important and 

 more constant characters of the vegetative organs, or of the 

 flowers and fruits, or both. When the differences become 

 still more pronounced, important, and constant, we agree to 

 recognise distinct genera a genus being of higher rank than 

 a species just as a species ranks above a variety. A number 

 of related genera, agreeing in certain broad features, though 

 differing (often considerably) in others, constitute a Natural 

 Order. 



Moreover, though different Natural Orders differ from one 

 another in important features, we can often trace a general 

 likeness in a few characters, enabling us to group the Orders 

 into Cohorts. 



