108 ESSAY ON CLASSIFICATION. Part I. 



illustrate this proposition, through the whole animal kingdom, as such an attempt 

 would involve the assertion that I know all these relations, or that where there 

 exists a discrepancy between the classification and the succession of animals, the 

 classification must be incorrect, or the relationship of the fossils incorrectly appre- 

 ciated. I shall therefore limit myself here to a general comparison, which may, 

 however, be sufficient to show, that the improvements which have been introduced 

 in our systems, upon purely zoological grounds, have nevertheless tended to render 

 more apparent the coincidence between the relative standing among living animals 

 and the order of succession of their representatives in past ages. I have lately 

 attempted to show, that the order of Halcyonoids, among Polyps, is superior to that 

 of Actinoids ; ^ that, in this class, compound communities constitute a higher degree of 

 development, when contrasted with the characters and mode of existence of smgle 

 Polyps, as exhibited by the Actinia; that top-budding is superior to lateral budding; 

 and that the type of Madrepores, with their top-animal, or at least with a defi- 

 nite and limited number of tentacles, is superior to all other Actmoids. If this be 

 so, the prevalence of Actinoids in older geological formations, to the exclusion of 

 Halcj'onoids, the prevalence of Bugosa and Tuhulafa in the oldest deposits,^ the 

 later prevalence of Astra?oids, and the very late introduction of Madrepores, would 

 already exhibit a correspondence between the rank of the living Polyps and the 

 representatives of that class in past ages, though we may hardly expect a very close 

 coincidence in this respect between animals the structure of which is so simple. 

 The gradation among the orders of Echinoderms is perfectly plam. Lowest 

 stand the Crinoids, next the Asterioids, next the Echinoids, and highest the Holo- 

 thurioids. Ever since this class has been circumscribed within its natural limits, 

 this succession has been considered as expressing their natural relative standing, and 

 modern investigations respecting their anatomy and emljryology, however extensive, 

 have not led to any important change in their classification, as far as the estimation 

 of their rank is concerned. This is also jjrecisely the order in which the representar 

 tives of this class have successively been introduced upon earth in past geological ages. 

 Among the oldest formations we find pedunculated Cinoids^ only, and this order 

 remains prominent for a long series of successive periods ; next come free Crinoids 

 and Asterioids; next Echinoids,* the successive appearance of which since the triasic 



^ For classification of Polypi, see Dana, q. a., p. * See the works q. a., p. 9G ; also : Muller, (.J.,) 



31, note 2 ; also Milne-Edwards and Haimk, q. a., .nnd Tkoschel, (F. II.,) System der Asteriden, 



and Agassiz, (L.,) Classitication of Polyps, Proc. Braunschweig, 1842, 4to. fig. — Muller, (J.,) Ueber 



Am. Acad. Sc. and Arts, 1856, p. 187. den Bau der Echinodermen, Berlin, 18.54, 4to. — Tie- 



^ See Milne-Edwards and Haime, q. a., p. 31. deman, (Fr.,) Anatomie der Rohren-Holothurie, des 



" Miller, Crinoids, q. a. — D'Orbigny, q. a. — Seeigels, etc., Landshut, 1817, fol. fig. — Valentin, 



J. Hall, q. a. — Austin, q. a., p. 90. (G.,) Anat. du gerne Echinus, Neuchatel, 1842, 4to. 



