86 • ESSAY ON CLASSIFICATION. Part I. 



among Mollusks, under the name of Cii-ripeds. It was not until Thompson ^ had 

 shown, what was soon confirmed by Burmeister and Martin St. Ange, that the 

 young Barnacle has a structure and form identical with that of some of the most 

 common Entomostraca, that their true position in the system of animals could be 

 determined; when they had to be removed to the class of Crustacea, among Articu- 

 lata. The same was the case with the Lernajans, which Cuvier arranged with the 

 Intestinal Worms, and which Nordmann has shown upon embryological evidence to 

 belong also to the class of Crustacea.^ Lamarck associated the Crinoids with Polypi, 

 and though they were removed to the class of Echinoderms by Cuvier, before the 

 metamorphoses of the Comatula were known,'^ the discovery of their jiedunculated 

 young furnished a direct proof that this was their true position. 



Embryology affords further a test for homologies in contradistinction of analogies. 

 It shows that true homologies are limited respectively within the natural boundaries 

 of the great branches of the animal kingdom. 



The distinction between homologies and analogies, upon which the English natu- 

 ralists have first insisted,* has removed much doubt respecting the real affinities of 

 animals which could hardly have been so distinctly appreciated before. It has 

 taught us to distinguish between real affinity, based upon structural conformity, and 

 similarity, based upon mere external resemblance in form and habits. But even after 

 this distinction had been ftxirly established, it remained to determine within what 

 liiuits homologies may be traced. The works of Oken, Spix, Geoffroy, and Carus,^ 

 show to what extravagant comparisons a preconceived idea of unity may lead. It 

 was not until Baer had shown that the development of the four great bi'anches of 

 the animal kingdom is essentially different,'^ that it could even be suspected that 

 organs performing identical functions may be different in their essential relations to 

 one another, and not until Rathke" had demonstrated that the yolk is in open 

 communication with the main cavity of the Articulata, on the dorsal side of the 

 animal, and not on the ventral side, as in Vertebrata, that a sohd basis was ob- 

 tained for the natural limitation of true homologies. It now appears more and 

 more distinctly, with every step of the progress Embryology is making, that the 

 structure of animals is only homologous within the limits of the four great branches 



^ Thompson's Zool. Kesearches, etc. ; Burmeis- ^ See, above, Sect. IV., notes 1 and 2. 



tee's Beitriige, etc.; Martin St. Ange, Mem. sur ^ Baer's Entwiekelungsgesclnclite, vol. 1, p. 160 



I'organisation, etc., quoted above, page 79, note 1. and 224. The extent of Baer's information and the 



^ Nordmann's Micrograpliische Beytriige, q. a. comprehensiveness of his views, nowhere appear so 



' Thompson and Forbes, q. a., page 79. strikingly as in tliis part of his work. 



* Swainson's Geography and Classification, etc. ' Rathke's Unters. Uber Bild., etc., see, above, p. 



See above, Sect. V., p. 20. 79, note 2. 



