126 ESSAY ON CLASSIFICATION. 



Embryology has, however, a wider scope than to trace 

 the growth of individual animals, the gradual building up 

 of their body, the formation of their organs, and all the 

 changes they undergo in their structure and in their form ; 

 it ought also to embrace a comparison of these forms and 

 the successive steps of these changes between all the types 

 of the animal kingdom, in order to furnish definite stan- 

 dards of their relative standing, of their affinities, and of 

 the correspondence of their organs in all their parts. Em- 

 bryologists have thus far considered too exclusively the 

 gradual transformation of the egg into the perfect animal. 

 There remains still a wide field of investigation, to ascer- 

 tain the different degrees of similarity between the succes- 

 sive forms which an animal assumes until it has completed 

 its growth, and the various forms of different kinds of 

 full-grown animals of the same type ; between the differ- 

 ent stages of complication of their structure in general, 

 and the perfect structure of their kindred ; between the 

 successive steps in the formation of all their parts and the 

 various degrees of perfection of the parts of other groups ; 

 between the normal course of the whole development of 



etc., Act. Ac. Nat. Cur., vol. 10, p. Ornithorhynchus paradoxus, Phil. 

 139, fig. COSTE (P.), Embryogunie Trans. 1834, p. 555. Oil the Young 

 cornparee ; Paris, 1837, Svo. Atlas of the Ornithorhynchus paradoxus, 

 4to.. Histoire particuliere et gene- Trans. Zool. Soc., i. p. 221 ; Proc. 

 rale du developperuent des corps Zool. Soc., ii. p. 43 ; Ann. Sc. Nat., 

 organises, q. a. Recherches sur la 2d ser. ii. p. 303 ; iii. p. 299. On 

 generation des Mammiferes et le de- the Generation of the Marsupial Ani- 

 veloppement de la brebis, Ann. Sc. mals, etc., Phil. Trans., 1824, p. 333. 

 Nat. 1835, III., p. 78. Recherches On the Placenta of the Elephant, 

 sur la generation des Mammiferes ; Proc. Roy. Soc. ; London, 1857, p. 

 Paris, 1834, 4to. fig. BERXHARDT 471. MEIGS (Cn.), Observations on 

 (C. A.), Syrnbolse ad Ovi Mamma- the Reproductive Organs and on the 

 liuni historian! ante pregnationern, Foetus of jDeIphinusA T esarnak,3o\iTu. 

 Vratisl., 4to., Muller's Arch., 1835, Ac. Nat. Sc. Phil., new ser. 1849, vol. 

 p. 228. BARRY (M.), Researches in 1, p. 267. WYMAN (F.), On the con- 

 Embryology, Phil. Trans. R. Soc. nection between the Uterus and the 

 1838, p. 301 ; 1839, p. 307; 1840, p. Chorion in Pigs, Proc. Bost. Nat. 

 529 ; 1841, p. 195. BAER (H. E. v.), Hist. Soc., 1858. 

 q. a. OWEN (R.), On the Ova of the 



