70 BEITISH ASSOCIATION FOB 



generations and centuries before it was again fanned into brightness, and a clear 

 view regained, both of the extent of ancient discovery, and of the true course to 

 be pursued by modern research. Rapid and right has been the progress of Zoology 

 since that resumption. Wot only has the structure of the animal been investigated, 

 even to the minute characteristics of each 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, onward 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 ade- 

 quate and lasting basis for the higher axioms and generalizations of Zoology pro- 

 perly so called. Three principles, of the common ground of which we may ulti- 

 mately obtain a clearer insight, are now recognized to have governed the con- 

 struction of animals : — unity of plan, vegetative repetition, 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 series of Hke 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 iu 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 continued — 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 importance will be to determine the homo- 

 logous parts of the nervous system, of the muscular system, of the respiratory and 

 vascular system, and of the digestive, secretory and genei'ative organs in the same 

 primary group or pi evince. I think it of more importance to settle the homologies 

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

 Speculate on such relations of parts of animals constructed on demonstratively dis- 

 tinct plans of organization. What has been effected and recommended, 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 deter- 

 mined to a praiseworthy extent, throughout that province. The same study is 

 making progress in the MoUusea; but the grounds for determining special homo, 

 logics are less sure in this sub-kingdom. The present state of homology in regard 

 to the Articulata has sufficed to demonstrate that the segment of the crust is not 

 a hollow expanded homologue of the segment of the endo-skeleton of a vertebrate. 

 There is as little homology between the parts and appendages of the segments of 

 the Vertebrate and Articulate skeletons respectively. The parts called mandibles, 

 maxillte, arms, legs, wings, fins, in Insects and Crustaceans, are only " analogous' 



