56 KTIKNNK GKOl-THOY SAINT-IIILAIKK 



with his principal hypothesis of the invariability of con- 

 nections, and that he, so to speak, gets a hold on his fish 

 to apply his principle of connections only by admitting at the 

 very outset an exception to his primary principle. A further 

 application of the hypothesis of metastasis will be noticed 

 below in connection with the determination of the sternum 

 of fishes. We note here an interpretation of the first metas- 

 tasis in terms of functional adaptation. " The constant and 

 violent action of the tail, if it does not go so far as actually 

 to displace and move forward the internal organs, at least fits 

 in well with an arrangement in which the organs are so 

 disposed " (p. 99). 



The first memoir deals with the homologies of the 

 opercular bones. Geoffrey considers that the external 

 opening of the ear corresponds to the external opening of the 

 gill-chamber, which lies between the operculum and the 

 pectoral girdle. The ear communicates with the buccal 

 cavity by the Eustachian tube, so does the branchial chamber 

 by means of the gill-slits. The auditory chamber of higher 

 Vertebrates is, therefore, the homologue of the branchial 

 chamber in fish ; the opercular bones in fish and the ossicles 

 of the ear in other Vertebrates stand in close relation to this 

 chamber ; therefore the opercular bones are the homologues 

 of the ossicles of the ear, the interoperculum corresponding 

 to the malleus, the suboperculum to the lenticular, the 

 minute lower part of the suboperculum to the incus, the 

 operculum to the stapes, and the pre-operculum to the 

 tympanic ring. In making these particular determinations 

 Geoffroy professes to be led by his principle of connections. 

 The pre-operculum has, he says, the same connections with 

 neighbouring bones as the tympanic bone in other Verte- 

 brates, and the other pieces of the gill-cover are homologised 

 with particular ear-ossicles according to the order in which 

 they stand to one another. The second memoir in the book 

 deals with the sternum, and affords a very good example of 

 Geoffrey's method of dealing with the facts of structure. \\'e 

 shall omit here any detailed reference to the other three 

 memoirs, which deal with the hyoid, with the branchial 

 arches and the structures which correspond in air-breathing 

 Vertebrates, and with the bones of the shoulder-girdle. 



