CLASSIFICATION 457 



Our classifications are often plainly influenced by chains 

 of affinities. Nothing can be easier than to define a number 

 of characters common to all birds; but with crustaceans, any 

 such definition has hitherto been found impossible. There 

 are crustaceans at the opposite ends of the series, which have 

 hardly a character in common; yet the species at both ends, 

 from being plainly allied to others, and these to others, 

 and so onwards, can be recognised as unequivocally belonging 

 to this, and to no other class of the Articulata. 



Geographical distribution has often been used, though 

 perhaps not quite logically, in classification, more especially 

 in very large groups of closely allied forms. Temminck in- 

 sists on the utility or even necessity of this practice in certain 

 groups of birds; and it has been followed by several ento- 

 mologists and botanists. 



Finally, with respect to the comparative value of the vari- 

 ous groups of species, such as orders, sub-orders, families, 

 sub-families, and genera, they seem to be, at least at present, 

 almost arbitrary. Several of the best botanists, such as Mr. 

 Bentham and others, have strongly insisted on their arbi- 

 trary value. Instances could be given amongst plants and 

 insects, of a group first ranked by practised naturalists 

 as only a genus, and then raised to the rank of a sub-family 

 or family; and this has been done, not because further re- 

 search has detected important structural differences, at 

 first overlooked, but because numerous allied species with 

 slightly different grades of difference, have been subse- 

 quently discovered. 



All the foregoing rules and aids and difficulties in classifi- 

 cation may be explained, if I do not greatly deceive myself, 

 on the view that the Natural System is founded on descent 

 with modification; — that the characters which naturalists 

 consider as showing true affinity between any two or more 

 species, are those which have been inherited from a common 

 parent, all true classification being genealogical : — that com- 

 munity of descent is the hidden bond which naturalists have 

 been unconsciously seeking, and not some unknown plan of 

 creation, or the enunciation of general propositions, and the 

 mere putting together and separating objects more or less 

 alike. 



