240 STUDY OF NATURAL HISTORY. 



the group before him is distinguished from all others ; 

 and if he can do this effectually, it matters not by 

 what means the object is accomplished. He is not, 

 however, to expect that he can so far isolate a 

 natural group, as that there shall be no exceptions 

 to the characters he assigns it ; or that each of the 

 individuals composing it shall possess those charac- 

 ters in the same degree. This would imply not only 

 the existence of absolute divisions in nature, — 

 which all experience is opposed to, — but would be 

 directly at variance with what has been just said on 

 the chain of continuity or gradation; for where 

 there is gradation in structure, there must be gra- 

 dation in character. It will be sufficient, if the 

 greater portion accord with his definition, and if the 

 others present a gradual diminution of the same set 

 of characters, fading and blending into others be- 

 longing to adjoining groups. In searching, there- 

 fore, for such characters, we must take into the 

 account every circumstance that is known regarding 

 the economy and the structure of the objects them- 

 selves ; and from all these make a selection of such 

 as are most constant, universal, and obvious. It 

 will almost always be found, that a peculiarity of 

 internal organisation is accompanied by a corre- 

 sponding difference in external structure, and that 

 both these are adapted for that particular mode of 

 life which the animal pursues. As there is a con- 

 stant harmony between the conformation of an 

 animal and its peculiar economy, we should study 

 the former with a constant reference to the latter, 

 but yet draw our characters from the first rather 

 than from the second ; because form can be always 



