346 CLASSIFICATION. [CHAP. XIV. 



see why these characters possess so high a value in classification, 

 namely, from the natural system being genealogical in its arrange- 

 ment. 



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 crusta- 

 ceans 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 insists on the utility 

 or even necessity of this practice in certain groups of birds ; and 

 it has been followed by several entomologists and botanists. 



Finally, with respect to the comparative value of the various 

 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 arbitrary value. Instances could 

 be given amongst plants a,nd 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 research has detected important structural differences, 

 at first overlooked, but because numerous allied species with 

 slightly different grades of difference, have been subsequently 

 discovered. 



All the foregoing rules and aids and difficulties in classification 

 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 community 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. 



But I must explain my meaning more fully. I believe that the 

 arrangement of the groups within each (jlass, in due subordination 

 and relation to each other, must be strictly genealogical in order 

 to be natural; but that the amount of difference in the several 

 branches or groups, though allied in the same degree in blood to 

 their common progenitor, may differ greatly, being due to the 



