xxx.] CLASSIFICATION. 709 



most readily be observed. Thus the two great classes of 

 phanerogamous plants are denned respectively by the 

 possession of two cotyledons or seed-leaves, and one coty- 

 ledon. But when a plant comes to our notice and we 

 want to refer it to the right class, it will often happen 

 that we have no seed at all to examine, in order to dis- 

 cover whether there be one seed-leaf or two in the germ. 

 Even if we have a seed it will often be small, and a careful 

 dissection under the microscope will be requisite to ascer- 

 tain the number of cotyledons. Occasionally the examina- 

 tion of the germ would mislead us, for the cotyledons may 

 be obsolete, as in Cuscuta, or united together, as in Clin- 

 tonia. Botanists therefore seldom actually refer to the 

 seed for such information. Certain other characters of a 

 plant are correlated with the number of seed-leaves ; thus 

 monocotyledonous plants almost always possess leaves with 

 parallel veins like those of grass, while dicotyledonous 

 plants have leaves with reticulated veins like those of an 

 oak leaf. In monocotyledonous plants, too, the parts of the 

 flower are most often three or some multiple of three in 

 number, while in dicotyledonous plants the numbers four 

 and five and their multiples prevail. Botanists, therefore, 

 by a glance at the leaves and flowers can almost certainly 

 refer a plant to its right class, and can infer not only the 

 number of cotyledons which would be found in the seed or 

 young plant, but also the structure of the stem and other 

 general characters. 



Any conspicuous and easily discriminated property 

 which we thus select for the purpose of deciding to which 

 class an object belongs, may be called a characteristic. The 

 logical conditions of a good characteristic mark are very 

 simple, namely, that it should be possessed by all objects 

 entering into a certain class, and by none others. Every 

 characteristic should enable us to assert a simple identity ; 

 if A is a characteristic, and B, viewed intensively, the class 

 of objects of which it is the mark, then A = B ought to be 

 true. The characteristic may consist either of a single 

 quality or circumstance, or of a group of such, provided 

 that they all be constant and easily detected. Thus in the 

 classification of mammals the teeth are of the greatest 

 assistance, not because a slight variation in the number 

 and form of the teeth is of importance in the general 



