84 ESSAY ON CLASSIFICATION. Part I. 



Embryology lias, however, a wider scope than to ti'ace 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 standards 

 of their i-elative standing, of their affinities, of the correspondence of their organs in 

 all their j^arts. Embryologists have thus far considered too exclusively, the gradual 

 transformation of the egg into a perfect animal ; there remains still a Avide field of 

 investigation to ascertain the different degrees of similarity between the successive 

 forms 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 different stages 

 of com2)lication 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 ])arts of other groups ; between the normal 

 course of the whole development of one type compared with that of other types, as 

 well as between the ultimate histological differences wliich all exhibit within certain 

 limits. Though important fragments have been contributed upon these different 

 points, I know how much remains to be done, from the little I have as yet been 

 able to gather myself, by systematic research in this direction. 



I have satisfied myself long ago, that Embryology furnishes the most trustworthy 

 standard to determine the relative rank among animals. A careful comparison of 

 the successive stages of development of the higher Batrachians furnishes, perhaps, the 

 most striking example of the importance of such investigations. The earlier stages 

 of the Tadpole exemplify the structure and form of those Ichthyoids which have 

 either no legs, or very imperfect legs, with and without external gills ; next it 

 assumes a shape reminding us more of the Tritons and Salamanders, and ends with 

 the structure of the Frog or Toad.^ A comparison between the two latter families 

 might prove further, that the Toads are higher than the Frogs, not only on account 

 of their more terrestrial habits (see Sect. 16), but Ijecause the embryonic web, which, 

 to some extent, still unites the fingers in the Frogs, disappears entirely in the Toads, 

 and may be also, because glands are developed in their skin, which do not exist in 

 Frogs. A similar comparison of the successive changes of a new species of Comatula 

 discovered by Prof Holmes, in the harbor of Charleston, in South Carolina, has 

 shown me in what relation the different types of Crinoids of past ages stand to 



doxus, Trans. Zool. Soc, i. p. 221 ; Proc. Zool. Soc, (Cil.,) Observations on the Eeproduetive Organs and 



ii. p. 43; Ann. Sc. Nat., 2d ser. ii. p. 303; iii. on tlie Foetus of Delphinus Nesarnak, Journ. Ac. 



p. 209. — On the Generation of the Marsupial Ani- Nat. Sc. Phil., new ser. 1849, vol. 1, p. 2G7. 

 mals, etc., Phil. Trans., 1824, p. 333. — Meigs, ^ Agassiz, (L.,) Twelve Lectures, etc., page 8. 



