Chap. XIV.] CLASSIFICATION. 369 



hitherto been found impossible. There are crustaceans at the 

 opposite ends of the series, which have hardly a character in com- 

 mon ; 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 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 

 research has detected important structural differences, at first over- 

 looked, 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 in- 

 herited from a common parent, all true classification being genea- 

 logical; — 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 class, 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 

 different degrees of modification which they have undergone ; and 

 this is expressed by the forms being ranked under different genera, 

 families, sections, or orders. The reader will best understand what 

 is meant, if he will take the trouble to refer to the diagram in the 

 fourth chapter. We will suppose the letters A to L to represent 



2 B 



