84 



THE GENESIS OF SPECIES. 



[CHAP. 



utterly remote from that on which the animals of the 

 higher division provided with a spinal column are con- 

 structed. And indeed no transitional form (tending even 

 to bridge over the chasm between those two groups) has 

 ever yet been discovered, either living or in a fossilized 

 condition. 1 



CUTTLE-FISH. 



A. Ventral aspect. B. Dorsal aspect. 



Nevertheless, in the two-gilled Cephalopods (Dibran- 

 ckiata) we find the brain supported and protected by a 

 cartilaginous cranium. In the base of this cranium are 

 two cartilaginous chambers. In each chamber is a mem- 

 branous sac containing an otolith, and the auditory nerves 

 pass from the cerebral ganglia into the cartilaginous cham- 

 bers to reach the auditory sacs. Moreover, it has been 



1 This remark is made without prejudice to possible affinities in the 

 direction of the Ascidians, an affinity which, if real, would be irrelevant 

 to the question here discussed. 



