CLASSIFICATION. 435 



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 onward, can be recognized as 

 unequivocally belonging to this, and to no other class of 

 the Arbiculata. 



Geographical distribution has often been used, though 

 perhaps not qaite logically, in classification, more espec- 

 ially in very large groups of closely allied forms. Tem- 

 •minck insists on the utility or even necessity of this 

 practice in certain groaps of birds; and it has been fol- 

 lowed by several entomologists and botanists. 



Finally, with respect to the comparative value of the 

 various groups of species, such as orders, suborders, fam- 

 ilies, subfamilies, 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 among 

 plants and insects, of a group first ranked by practiced 

 naturalists as only a genus, and then raised to the rank of 

 a subfamily or family; and this has been done, not be- 

 cause further research has detected important structural 

 differences, at first overlooked, but because numerous allied 

 species, with slightly difEerent grades of deference, have 

 been subsequently^ discovered. 



All the foregoing rules and aids and difficulties in clas- 

 sificfdiion may be explained, if I do not greatly deceive 

 myso^, 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 geuea, 

 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 



