162 SYSTEMATIC BOTANY. 



natural genera leads to the establishment of other genera wherein the 

 species seem at first sight to differ widely, of which we could not have a 

 better example than in the genus Euphorbia, where our native species are 

 inconspicuous herbs, while the tropics afford species with large spiny 

 Cactus-like trunks, &c. 



Moreover the carrying out of the same principle leads in certain cases 

 to the generic separation of species which present close agreement in 

 their general characters, but are distributable into a number of groups 

 characterized by very decided morphological diversities in important 

 parts of their floral organs. Thus, in the Umbelliferse, the Compositse, the 

 Grasses, and some other families, we separate generically species which 

 have a great resemblance in the maj ority of their characters. This happens 

 especially in what are called very natural families of plants, large assem- 

 blages of genera so evidently connected with each other by the presence 

 of some very marked peculiarity, such as the Umbelliferous inflorescence, 

 the Papilionaceous corolla of the Leguminosas, the Capitulous inflores- 

 cence of the Composite, the peculiar spikelets in the Grasses, &c., that no 

 doubt can be entertained as to their lineage. On the other hand, the 

 " natural genera " occur mostly where the character of the natural family 

 is more lax and flexible, as in the Ranunculaceae, Rosaceae, &c. 



In the present state of knowledge it must be admitted that a very 

 large portion of our generic distinctions are arbitrary, and that the 

 species included in some genera agree together much more closely than 

 those combined under other generic heads. At the same time it cannot 

 be doubted that some genera are really far more extensively represented 

 by species than others ; so that the mere number of kinds included in a 

 genus is to be totally neglected in a natural classification ; and many recent 

 authors have done disservice to science in general by splitting up large 

 natural genera on slight characters for the convenience of systematists. 

 It is far more instructive to keep together the members of large natural 

 genera, like Ficus, Erica, Begonia, &c., than to subdivide them under 

 names which disguise their relations ; and the convenience of systematists 

 may always be sufficiently regarded by the establishment of sections in 

 extensive descriptive works. 



Genera are groups of species associated on account of agreement 

 in the essential characters of their floral organs ; but here, as else- 

 where in nature, variations from our abstract types must be ad- 

 mitted. Some undoubtedly natural genera include species with 

 their floral organs varying in certain particulars more than is usual 

 in groups associated under a common type, somewhat as certain 

 species admit of a wider range of variation than others. Here, 

 again, physiological characters become of value ; and as in species 

 we regard the fertility of the seeds produced by unlimited cross- 

 breeding between the varieties as a proof of these being individuals 

 of the same species, so with regard to genera it is commonly held 

 that a generic connexion between diverse species is indicated by 

 the capability of producing hybrids by cross-breeding. These true 

 hybrids produced between distinct species of the same genus are 

 often barren, or only breed with individuals of one of the parent 



