III.] INDEPENDENT SIMILARITIES OF STRUCTURE. 85 



suggested by Professor Owen that sinuosities between pro- 

 cesses projecting from the inner wall of each chamber 

 " seem to be the first rudiments of those which, in the higher 

 classes (^. e. in animals with a spinal column), are extended 

 in the form of canals and spiral chambers, within the sub- 

 stance of the dense nidus of the labyrinth."^ 



Here, then, we have a wonderful coincidence indeed ; 

 two highly complex auditory organs, marvellously similar 

 in structure, but wliich must nevertheless have been de- 

 veloped in entire and complete independence one of the 

 other I It would be difficult to calculate the odds asi-ainst 

 the independent occurrence and conservation of two such 

 complex series of merely accidental and minute haphazard 

 variations. And it cannot be maintained that the sense of 

 hearing could not be efficiently subserved otherwise than 

 by such sacs, in cranial cartilaginous capsules so situated 

 in relation to the brain, &c. 



Our wonder, moreover, may be increased when we recol- 

 lect that the two-gilled cephalopods have not yet been 

 found below the lias, where they at once abound ; whereas 

 the four-gilled cephalopods are Silurian forms. Moreover, 

 the absence is in this case significant in spite of the imper- 

 fection of the geological record ; because when we consider 

 how many individuals of various kinds of four-gilled 

 cephalopods have been found, it is fair to infer that at the 

 least a certain small percentage of dibranchs would also 

 have left traces of tlieir presence had they existed. Thus 

 it is probable that some four-gilled form was the progenitor 

 of the diljranch ceplialopods. jS'ow the four-gilled kinds 

 (judging from the only existing form, the nautilus) had the 



^"Lectures on the Comp. Anat. of the Invertebrate Animals," 2nd edit. 

 1855, p. 619 ; and Todd's " Cj^clopsedia of Anatomy," vol. i. p. ^^L 



