Chap. I. RANK AND DEVELOPMENT OF ANIMALS. 119 



fiirthor mention lioiv. It has alsu boon shown above (Sect, VIII.) that animals do 

 not form such a simple series as would result from a successive development. 

 There remains, therefore, only for us to show now Avithin what limits the natural 

 gradation which may be traced in the diflerent types of the animal kingdom,^ cor- 

 responds to the changes they undergo during their growth, having already considered 

 the relations which exist between these metamorphoses and the successive appear- 

 ance of animals upon earth, and betw^een the latter and the structural gradation or 

 relative standing of their living representatives. Our knowledge of the complication 

 of structure of all animals is sufficiently advanced to enable us to select, almost at 

 random, our examples of the correspondence between the structural gradation of 

 animals and their embryonic growth, in all those classes the embryologic develop- 

 ment of which has been sufficiently investigated. Yet, in order to show more 

 distinctly how closely all the leading features of the animal kingdom are combined, 

 whether we consider the complication of their structure, or their succession in time, 

 or their embryonic development, I shall refer by preference to the same t;y^es 

 which I have chosen before for the illustration of the other relations. 



Among Echinoderms, we find in the order of Crinoids the pedmiculated types 

 standing lowest,^ Comatula3 highest, and it is well known that the young Comatula 

 is a pedunculated Crinoid, which only becomes free in later life.^ J. Miiller has 

 showai that among the Echinoids, even the highest representatives, the Spatan- 

 goids, difier but slightly in early youth from the Echinoids, and no zoologist 

 can doubt that these are inferior to the former. Among Crustacea, Dana* has 

 insisted particularly upon the serial gradation w^hich may be traced between the 

 different types of Decapods, their order being naturally from the highest Bruchyoura, 

 through the Anomoura, the Macroura, the Tetradecapods, etc., to the Entomostraca ; 

 the Macrouran character of the embryo of our Crabs has been fidly illustrated 

 by Kathke,'' in his beautiful investigations upon the embryology of Crustacea. I 

 have further shown that the young of Macroura represents even Entomostraca 

 forms, some of these young having been described as representatives of that 

 order." The correspondence between the gradation of Insects and their embryonic 

 growtli, I ha\o illustrated fully in a special paper." Similar comparisons have been 

 nuule in the class of Fishes;^ among Reptiles, we fuid the most striking examples 



^ See the \voiks<iiiriic(l f'niiii p. (IT-HT, ;ils() INIii.NK- ■• D.vx.v, q. a., i>. .'32. — Burmeisteu, Cirripeds, 



Edwauds, ([. a., \). 11"-'. — Thompson, Criiioiils, q. a. q. a., p. 79. — Tiio.MrsON, q. a., p. 70. 



* MuLLEH, (J.,) Ucbcr reiitacrinus C'aput-Me- ' Rathke, q. a., p. 79. 

 (lusfe, Berlin, 1833, 4to., Ak. d. Wiss. ° Twelve Lei-tures, etc., p. 67. 



* Forbes, (Ed.,) History of British Starfishes, ' Classification of Insects, (j. a. 

 London, 1851, 1 vol. 8vo., p. 10. ' Poissons fossiles, q. a. 



