114 ESSAY ON CLASSIFICATION 



sion of the gigantic Reptiles of the middle geological ages, as I shall 

 show that the embryology of Turtles throws light upon the fossil 

 Chelonians. It is already plain that the embryonic changes of Batra- 

 chians coincide with what is known of their succession in past ages. 

 The fossil Birds are too little known, and the fossil Mammalia do 

 not extend through 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. Pedunculated Crinoids are embryonic 

 types of the Comatuloids, the oldest Echinoids embryonic represen- 

 tatives of the higher living families, Trilobites embryonic types of 

 Entomostraca, the Oolitic Decapods embryonic types of our Crabs, 

 the Heterocercal Ganoids embryonic types of the Lepidosteus, the 

 Andrias Scheuchzeri an embryonic prototype of our Batrachians, the 

 Zeuglodonts embryonic Sirenidae, the Mastodons embryonic Ele- 

 phants, 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 their whole organization early stages of 

 growth of higher representatives of the same type, and embryonic 

 features prevailing more or less extensively in the characters of allied 

 genera, as in the case of the Mastodon and Elephant, and what I would 

 call hypembryonic types, in which embryonic features are developed 

 to extremes in the further periods of growth, as, for instance, the 

 wings of the Bats, which exhibit the embryonic character of a webbed 

 hand, as all Mammalia have it at first, but here grown out and de- 



