SUCCESSION AND STANDING OF ANIMALS. 165 



to the Crustacea. 1 Now this gradation presents the 

 most perfect coincidence with the order of succession of 

 Crustacea in past geological ages, even down to their sub- 

 divisions into minor groups. Trilobites and Entomostraca 

 are the only representatives of the class in palaeozoic 

 rocks ; in the middle geological ages there appears a 

 variety of Shrimps, among which the Macrouraii Decapods 

 are prominent, and in the later only the Brachyura, which 

 are the most numerous in our days. 



The fragmentary knowledge which we possess of fossil 

 Insects does not justify us, yet, in expecting to ascertain 

 with any degree of precision the character of their suc- 

 cession through all geological formations, though much 

 valuable information has already been obtained respecting 

 the entomological faunae of several geological periods. 2 



The order of succession of Vertebrata in past ages 

 exhibits features in many respects differing greatly from 

 the Articulata, Mollusks, and Eadiata. Among the latter 

 we find the 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, Keptiles, Birds, and Mammalia are 

 introduced successively 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 con- 

 trary, 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. 26), to the exclusion of what henceforth constitutes 

 the special character of the lower class. For instance, 



1 LEYDIG, (FR.,) Riiderthiere, etc., thiere, Zeitsch. f. wiss. Zool. 1856, 



Zeitsch. f. wiss. Zool. 1854, vol. 6, p. vol. 8, p. 152. GOSSE, (Tn. H.,) On 



1. DANA, (J. D.,) Crustacea q. a. the Structure of the class Rotifera 



BURMEISTER, Noch einige Worte iiber q. a. 

 die systematischeStelluug dcr lliider- 2 HEEB, q. a.; BKODIE, q. a., p. 147. 



