III.] INDEPENDENT SIMILARITIES OF STRUCTURE. 80 



c.iiials and spiral cliatnbcrs, within the substance of the 

 dense nidus of the hibyrinth." " 



Here, then, we have a wonderful coincidence indeed ; 

 two highly-complex auditory organs, marvellousl}' similar 

 in structure, but which must nevertheless have been devel- 

 oped in entire and complete independence one of the 

 other I It woidd be difTicult to calculate the odds against 

 the indcjiendent occurrence and conservation of two such 

 complex series of merely accidental and minute haphazard 

 variations. And it can never be maintained that the sense 

 of hearing could not be eflicicntly subserved otherwise 

 than by snc^h sacs, in cranial cartilaginous capsules so situ- 

 ated in rclalion to the [)rain, etc. 



Our wonder, moreover, may be increased when we 

 recollect 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 cej)hal- 

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

 a certain small percentage of cHbranchs would also have 

 left traces of their presence had they existed. Thus it is 

 probable that sotuc four-gilled form was the progenitor of 

 the dil)ranch cephalopods. Now, the four-gilled kinds 

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

 auditory organ in a very inferior condition of development 

 to what we find in the dibranch ; thus we have not only 

 evidence of the independent high development of the organ 

 in the former, but also evidence pointing toward a certain 

 degree of comparative rapidity in its development. 



Such being the case with regard to the organ of hear- 

 ing, we have another yet stronger argument with regard to 



" " Lectures on llic Comp. Anat. of the Invertebrate Animnls," 2(1 

 edit., 1855, p. 619 ; and Todd's " Cyclopaedia of Anatomy," vol. i., p. 664. 



