The Vertebrates approach the Molluscs at the low- 

 est extreme of the former and higher of the latter. 

 The lamprey eels of the one possess several characters 

 in common with the cuttle-fish or squids of the latter. 

 The amphioxus is called the lowest Vertebrate, and 

 though it is nothing else, the definition of the division 

 must be altered to receive it ; it has no brain ! 



The lowest forms of the Molluscs and Articulates are 

 scarcely distinguishable from each other, so far as adhe- 

 sion to the " plan " is concerned, and some of the lat- 

 ter division are very near certain Echinodermata. As 

 we approach the boundary-lines of the two lowest divis- 

 ions, the approaches become equally close, and the boun- 

 daries very obscure. 



More instructive is the evidence of the relation of 

 "the subordinate classes of any one of these divisions. 

 The conditions of those organs or parts which define 

 classes exhibit a regular relation, commencing with 

 simplicity and ending with complication; first asso- 

 ciated with weak exhibitions of the highest functions of 

 the nervous system at the last displaying the most ex- 

 alted traits found in the series. 



For example : In the classes of Vertebrates we find 

 the lowest nervous system presents great simplicity 

 the brain cannot be recognized ; next (in lampreys), the 

 end of the nervous axis is subdivided, but scarcely ac- 

 cording to the complex type that follows. In fishes the 

 cerebellum and cerebral hemispheres are minute, and 

 the intermediate or optic lobes very large : in the rep- 

 tiles the cerebral hemispheres exceed the optic lobes, 

 while the cerebellum is smaller. In birds the cerebel- 

 lum becomes complex and the cerebrum greatly in- 

 creases. In mammals the cerebellum increases in com- 

 plexity or number of parts, the optic lobes diminish, 



