26 ANNUAL OF SCIENTIFIC DISCOVERY. 



tissue, but the mode of formation of such constituents of organs, and of 

 the organs themselves, has been pursued from the germ, bud, or egg, on- 

 ward to maturity and decay. To the observation of outward characters is 

 now added that of inward organization and developmental change, and 

 Zootomy, Histology, and Embryology, combine their results in forming an 

 adequate and lasting basis for the higher axioms and generalizations of 

 Zoology, properly so called. Three principles, of the common ground of 

 which we may ultimately obtain a clearer insight, are now recognized to 

 have governed the construction of animals, unity of plan, vegetative repe- 

 tition, and fitness for purpose. The independent series of researches by 

 which students of the articulate animals have seen, in the organs performing 

 the functions of jaws and limbs of varied powers, the same or homotypal 

 elements of a scries of like segments constituting the entire body, and by 

 which students of the vertebrate animals have been led to the conclusion, 

 that the maxillary, mandibular, hyoid, scapular, costal, and pelvic arches, 

 and their appendages sometimes forming limbs of varied powers, are also 

 modified elements of a series of essentially similar vertebral segments, 

 mutually corroborate their respective conclusions. It is not probable that a 

 principle which is true for Articulata should be false for Vertebrata : the less 

 probable since the determination of homologous parts becomes the more 

 possible and sure in the ratio of the perfection of the organization. 



After pointing out the distinction between Affinity, which indicates an 

 intimate resemblance, and Analogy, which indicates a remote one, he con- 

 tinued : The study of homologous parts in a single system of organs the 

 bones has mainly led to the recognition of the plan or archetype of the 

 highest primary group of animals, the Vertebrata. The next step of impor- 

 tance will be to determine the homologous parts of the nervous system, of 

 the muscular system, of the respiratory and vascular system, and of the 

 digestive, secretory, and generative organs, in the same primary group or 

 province. I think it of more importance to settle the homologics of the 

 parts of a group or animals constructed on the same general plan, than to 

 speculate on such relations of parts of animals constructed on demonstra- 

 tively distinct plans of organization. What has been effected and recom- 

 mended, in regard to homologous parts in the Vertebrata, should be followed 

 out in the Articulata and Mollusca. In regard to the constituents of the 

 crust or outer skeleton and its appendages in the Articulata, homological 

 relations have been studied and determined to a praiseworthy extent, 

 throughout that province. The same study is making progress in the Mol- 

 lusca; but the grounds for determining special homologies are less sure in 

 this sub-kingdom. The present state of homology in regard to the Articu- 

 lata has sufficed to demonstrate that the segment of the crust is not a hollow 

 expanded homologue of the segment of the endo-skcleton of a vertebrate. 

 There is as little homology between the parts and appendages of the seg- 

 ments of the Vertebrate and Articulate skeletons respectively. The parts 

 called mandibles, maxilla}, arms, legs, wings, fins, in Insects and Crusta- 

 ceans, are only " analogous " to the parts so called in Vertebrates. A most 

 extensive field of reform is becoming open to the homologist in that which 

 is essential to the exactitude of his science, a nomenclature equivalent to 

 express his conviction of the different relations of similitude. Most difficult 

 and recondite arc the questions in face of which the march of Homology is 

 now irresistibly conducting the philosophic observer; such, for instance, as 

 the following: Are the nervous, muscular, digestive, circulating, and geuer- 



