174 ESSAY ON CLASSIFICATION. 



their succession in past ages. 1 Fossil Birds are too little 



known, and fossil Mammalia 2 do not extend throuoii 



& 



a sufficiently long series of geological formations, to 

 afford many striking points of comparison ; yet, the 

 characteristic peculiarities of their extinct genera exhibit 

 everywhere indications that their living representatives in 

 early life resemble them more than they do their own 

 parents. A minute comparison of a young elephant with 

 any mastodon will show this most fully, not only in the 

 peculiarities of their teeth, but even in the proportion of 

 their limbs, their toes, etc. 



It may therefore be considered as a general fact, very 

 likely to be more fully illustrated as investigations cover 

 a wider ground, that the phases of development of all 

 living animals correspond to the order of succession of 

 their extinct representatives in past geological times. As 

 far as this goes, the oldest representatives of every class 

 may then be considered as embryonic types of their 

 respective orders or families among the living. Pedun- 

 culated Crinoids are embryonic types of the Comatuloids, 

 the oldest Ecliinoids embryonic representatives of the 

 higher living families, Trilobites embryonic types of Ento- 

 mostraca, the Oolitic Decapods embryonic types of our 

 Crabs, the Heterocercal Ganoids embryonic types of the 

 Lepidosteus, the Andrias Scheuchzeri an embryonic proto- 

 type of our Batrachians, the Zeuglodonts embryonic 

 SirenidaB, the Mastodonts embryonic Elephants, etc. 



To appreciate, however, fully and correctly all these 

 relations, it is further necessary to make a distinction 

 between embryonic types in general, which represent in 



1 See the works, q. a., p. 124, note siz (L.,) Zoological Character of 



Young Mammalia, Proc. Am. Ass. 



2 Cuv., Oss. foss., q. a.; also, AGAS- Adv. Sc.; Cambridge, 1849, p. 85. 



