326 OPERATIONS SUBSIDIARY TO INDUCTION. 



tribution of the series may most properly take place : 

 in what manner it should be divided into Orders, 

 Families, and Genera. 



The main principle of division must of course be 

 natural affinity; the classes formed must be natural 

 groups : and the formation of these has already been 

 sufficiently treated of. But the principles of natural 

 grouping must be applied, in subordination to the 

 principle of a natural series. The groups must not 

 be so constituted as to place in the same group things 

 which ought to occupy different points of the general 

 scale. The precaution necessary to be observed for 

 this purpose is, that the primary divisions must be 

 grounded not upon all distinctions indiscriminately, 

 but upon those which correspond to variations in the 

 degree of the main phenomenon. The series of Ani- 

 mated Nature should be broken into parts at the 

 exact points where the variation in the degree of 

 intensity of the main phenomenon (as marked by its 

 principal characters, Sensation, Thought, Voluntary 

 Motion, &c.) begins to be attended by conspicuous 

 changes in the miscellaneous properties of the animal. 

 Such well marked changes take place, for example, 

 where the class Mammalia ends ; at the points where 

 Fishes are separated from Insects, Insects from Mol- 

 lusca, &c. When so formed, the primary natural 

 groups will compose the series by mere juxtaposition, 

 without redistribution; each of them corresponding 

 to a definite division of the scale. In like manner 

 each family should, if possible, be so subdivided, that 

 one portion of it shall stand higher and the other 

 lower, though of course contiguous, in the general 

 scale; and only when this is impossible is it allowable 

 to ground the remaining subdivisions upon characters 

 having no determinable connexion with the main 

 phenomenon. 



