CLASSIFICATION. 13 



in the Silurian, the Amphibians in the Carboniferous, 

 the Reptiles in the Permian, the Mammals in the Rhsetic. 

 The Birds are first found in the Solenhofen Slate (Upper 

 Jurassic), but they must be left out of account, since 

 owing to their mode of existence they rarely become 

 preserved. And similarly in each of these smaller groups, 

 the Fishes for instance, the genera of the earlier formations 

 are simpler and more generalized than those in the later. 



The animal kingdom may be divided into groups, the 

 primary groups are termed sub-kingdoms, the members of 

 each of these agreeing in the general arrangement of their 

 parts. The sub-kingdoms are divided into classes, the 

 classes into orders, and the orders into families. The 

 animals included in each possess certain characters in 

 common, which are not found in the other groups. The 

 families are composed of genera, and the genera of 

 species. The sub-kingdoms are: 1. Protozoa. 2. Porifera. 

 3. Ccelenterata. 4. Echinodermata. 5. Vermes. 6. Mol- 

 luscoidea. 7. Mollusca. 8. Arthropoda. 9. Vertebrata. 

 But the student must be warned against considering 

 that this order of succession represents an increasing per- 

 fection of organisation. Some of the members of one sub- 

 kingdom may be more highly developed than the lower 

 members of another which as a whole occupies a higher 

 position in the zoological scale. In other cases, owing to 

 the difference in the general arrangement of the parts, it 

 is difficult to determine which of two divisions is the 

 higher. 



