1412 CRUSTACEA. 



In order to compare allied animals of different sizes, it should be 

 noted, that while there is some foundation for the conclusion, that 

 under certain limitations, size is a mark of grade, rapidity of move- 

 ment or action should also be considered ; and the more proper com- 

 parison would be between multiples of size and activity. This deduc- 

 tion, is, however, true only in the most general sense, and rather 

 between species of allied groups than those of different types. We 

 may occasionally find something like an exemplification of the law 

 among bipeds, ludicrous though the idea may be. 



VI. We observe with regard to the passage in Crustacea to inferior 

 grades under a given type, that there are two methods by which it 

 takes place. 



1. A diminution of centralization, leading to an enlargement of the 

 circumference or sphere of growth at the expense of concentration, as 

 in the elongation of the antennae and a transfer of the maxillipeds to 

 the foot series, the elongation of the abdomen and abdominal appen- 

 dages, etc. 



2. A diminution of force as compared with the size of the structure, 

 leading to an abbreviation or obsolescence of some circumferential 

 organs, as the posterior thoracic legs or anterior antennae, or the abdo- 

 minal appendages (where such appendages exist in the secondary type 

 embracing the species). These circumstances, moreover, are indepen- 

 dent of a degradation of intelligence, by an extension of the sphere 

 of growth beyond the proper limits of the sphere of activity. 



VII. A classification by grades, analogous to that deduced for Crus- 

 tacea, may no doubt be laid out for other classes of animals. But the 

 particular facts in the class under consideration, are not to be forced 

 upon other classes. Thus, while inferiority among Crustacea is con- 

 nected with a diminished number of annuli cephalically absorbed (for 

 the senses and mouth), it by no means follows, that the Insecta, 

 which agree in the number of cephalic annuli with the lower Crustacea, 

 are allied to them in rank, or inferior to the higher species. On 

 the contrary, as the Insecta pertain to a distinct division, being aerial 

 instead of aqueous animals, they can be studied and judged of, only 

 on principles deduced from comparison among insects themselves. 

 They are not subject to Crustacean laws, although they must exem- 

 plify beyond doubt, the fundamental idea at the basis of those laws. 



The views which have been explained, lead us to a modification, in 

 some points, of the classification of Crustacea, adopted in the early 



