108 ESSAY ON CLASSIFICATION 



divisions into minor groups. Trilobites and Entomostraca are the 

 only representatives of the class in palaeozoic rocks; in the middle geo- 

 logical ages appear a variety of Shrimps, among which the Macrouran 

 Decapods are prominent, and later only the Brachyoura, which are 

 the most numerous in our days. 



The fragmentary knowledge we possess of the fossil Insects does 

 not justify us yet in expecting to ascertain with any degree of pre- 

 cision the character of their succession through all geological forma- 

 tions, though much valuable information has already been obtained 

 respecting the entomological faunae of several geological periods. 



The order of succession of Vertebrata in past ages exhibits features 

 in many respects differing greatly from the Articulata, Mollusks, and 

 Radiata. Among these we find their respective classes appearing 

 simultaneously in the oldest periods of the history of our earth. Not 

 so with the Vertebrata, for though Fishes may be as old as any of the 

 lower classes. Reptiles, Birds, and Mammalia are introduced succes- 

 sively in the order of their relative rank in their types. Again, the 

 earliest representatives of these classes do not always seem to be the 

 lowest; on the contrary, they are to a certain extent and in a certain 

 sense the highest, in as far as they embody characters which in later 

 periods appear separately in higher classes (see Sect, xxvi) to the ex- 

 clusion of what henceforth constitutes the special character of the 

 lower class. For instance, the oldest Fishes known partake of the 

 characters which at a later time are exclusively found in Reptiles 

 and no longer belong to the Fishes of the present day. It may be said 

 that the earliest Fishes are rather the oldest representatives of the 

 type of Vertebrata than of the class of Fishes, and that this class as- 

 sumes only its proper characters after the introduction of the class 

 of Reptiles upon earth. Similar relations may be traced between the 

 Reptiles and the classes of Birds and Mammalia, which they precede. 

 I need only allude here to the resemblance of the Pterodactyli and 

 the Birds, and to that of Ichthyosauri and certain Cetacea. Yet 

 through all these intricate relations there runs an evident tendency 

 towards the production of higher and higher types, until at last Man 

 crowns the whole series. Seen as it were at a distance, so that the 

 mind can take a general survey of the whole and perceive the con- 

 nection of the successive steps without being bewildered by the de- 

 tails, such a series appears like the development of a great conception. 



