MUTUAL AFFINITIES OF ORGANIC BEINGS 377 



number and position of the cotyledons, and on the mode of devel- 

 opment of the plumule and radicle. We shall immediately see 

 why these characters possess so high a value in classification; 

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

 rangement. 



Our classifications are often plainly influenced by chains of 

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

 acters common to all birds ; but with crustaceans, any such defini- 

 tion 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 onward, can be recognized 

 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-fam- 

 ilies, 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. In- 

 stances could be given among 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 overlooked, but because numerous allied spe- 

 cies, with slightly different grades of difference, have been sub- 

 sequently 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 be- 

 ing 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 proposi- 

 tions, and the mere putting together and separating objects more 

 or less alike. 



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



