FUNDAMENTAL RELATIONS OF ANIMALS 87 



Embryology affords further a test for homologies in contradistinc- 

 tion of analogies. It shows that true homologies are limited respec- 

 tively Avithin the natural boundaries of the great branches of the 

 animal kingdom. 



The distinction between homologies and analogies, upon which 

 the Enolish naturalists 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 be- 

 tween real affinity based upon structural conformity, and similarity 

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

 after this distinction had been fairly established, it remained to de- 

 termine within what limits homologies may be traced. The works 

 of Oken, Spix, Geoffroy and Carus,^^^ show to what extravagant com- 

 parisons a preconceived idea of unity may lead. It was not until von 

 Baer had shown that the development of the four great branches of 

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

 suspected that organs performing identical functions may be differ- 

 ent 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 solid 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 



^'^ Swainson, Geography and Classification of Animals. 



^*With reference to this point, consult: Lorenz Oken, Ueber die Bedeutung der 

 Schddel-Knochen (Frankfurt, 1807), pamphlet, and Lehrbuch der Naturphilosophie (3 

 vols., Jena, 1809-1811; tr., A. Tulk, London, 1847); Spix, Cephalogenesis, sive capitis 

 ossei structura, formatio et significatio (Munich, 1815); fitienne Geoffroy St. Hilaire, 

 Philosophic anatomique (2 vols., Paris, 1818-1823); Carus, Von den Ur-Theilen des 

 Knochen-und Schalengeriistes (Leipzig, 1828); Owen, On the Archetype and Homo- 

 logies of the Vertebrate Skeleton (London, 1848); Cuvier, "Sur un nouveau rapproche- 

 ment h. ^tablir entre les Classes qui composent le R^gne animal," Annales du Museum 

 d'Histoire Naturelle, XIX (1812), 73; von Baer, Entrvickelungsgeschichte; Leuckart, 

 Ueber die Morphologic und die Verwandtschaftsverhdltnisse der wirbellosen Thiere 

 (Brunswick, 1848); Agassiz, Twelve Lectures on Embryology. 



^^ Von Baer, Entivickelungsgeschichte, I, 160, 224. The extent of von Baer's informa- 

 tion and the comprehensiveness of his views nowhere appear so strikingly as in this 

 part of his work. 



"^ Martin Heinrich Rathke, Untersuchungen iiber die Bildung und Entwickelung des 

 Flusskrebses (Leipzig, 1829), and Beitrdge zur vergleichenden Anatomic und Physiologic, 

 Reisebemerkungen aus Skandinavien (Danzig, 1842). 



