based on the Principle of Geijlialization. 



Til 



gamniati/pic (or D and y), although the gammatypic is nearer in 

 grade to the degradational. — Thus, the line between Myriapods 

 and Insects is more distinct than that between Myriapods and 

 Spiders ; or that between Entomostracans and Decapods, than that 

 between Entomostracans and Tetradecapods. 



There is an exception in the class of Mammals : the Obtocoids 

 seem to graduate towards both Microsthenes and Megasthenes 

 with nearly equal distinctness. 



3. Co-ordinate grades and distinctions in Classification. 



X. The co-ordinate value of subdivisions in the system of classi- 

 fication is brought out to view in the parallel columns of the pre- 

 ceding tables, and evidence is thence afforded as to what groups 

 are rightly designated classes, orders, &c. 



a. We thus learn that the subdivisions of the class of Mammals 

 — Man, Megasthenes, Microsthenes — are properly orders, if we so 

 call the subdivisions Decapods and Tetradecapods under Crusta- 

 ceans, or Insects and Spiders under Insecteans. 



6. Again, we have a solution of the question whether in each 

 of the classes. Mammals, Birds, and Eeptiles, the hemitypic divi- 

 sion, as so-called on page 76, is a subclass co-ordinate with the 

 typiccd division of the same, or whether it is an order co-ordinate 

 with the three higher subdivisions of the class. The question 

 appears to be decided (contrary to former views of the writer), 

 that it is correctly made an order. These hemitypic divisions 

 actually correspond severally to the degradational division in 

 other columns of the different tables ; and, therefore, if in the 

 case of other classes as those of Crustaceans, Insecteans, &c., 

 they are orders, so are they in the tliree classes of Vertebrates 

 mentioned. They have also a relation to the hemitypic divisions 

 among Fishes, which are the first and second orders of the class. 



XI. In an inferior or degradational group, the distinctions of 

 the subdivisions included are generally much more strongly and 

 obviously exhibited in the structure than among typical groups. 

 Thus, the orders of Fishes are based on characters that have 

 nearly a class-value among the higher Vertebrates. In the same 

 manner. Amphibians, or hemitypic Reptiles, differ from true Rep- 

 tiles more obviously than Ootocoids, or hemitypic Mammals, 

 difier from other Mammals. So, the distinctions among the 

 groups of Crustaceans are very wide compared with those among 

 Insects ; and those among degradational Crustaceans far wider 

 than those among the typical subdivisions. The relative force of 

 the life- systems is, in all probability, as great between Ootocoids 

 and typical Mammals as between Amphibians and typical Reptiles, 

 although so unequally expressed in the structure of the high or 

 concentrated groups and the low or lax groups of species. Over- 



