184 AFFINITIES AND GEOGRAPHICAL 



^ cephalopods, though so highly organized in comparison 

 with the gasteropods, do not advance, like these, to land forms, 

 with^ apparatus for aerial respiration. They are, as a class, 

 restricted to a pelagic life, admitting of occasional appearances 

 on the surface of the ocean. Their respiratory system is 

 accordingly branchiate, yet with marks of grade which are 

 worthy of observation. It is, in the words of Professor Owen, 

 a law determining animal rank, that " increased number [of 

 parts] irrespective of correlative structure, in an organ of the 

 animal body, is ever a mark of its inferiority/' By this test, 

 the nautilus, with its four branchiae, sinks below the belemnite 

 and the cuttle-fish with only two ; and such is the basis of a 

 division of the cephalopoda. In the whole of this order, how- 

 ever, there is a remarkable advance of the nervous system, 

 though only to the effect of enabling the animal to supply 

 itself with food by conquest over the inferior tribes. The 

 nervous centres, which in lower mollusca were only protected 

 by coverings which also served to cover the rest of the body, 

 now become of sufficient importance to have a special protec- 

 tion, in the form of cartilaginous plates, which naturalists 

 interpret as the rudiment of an internal skeleton. In this 

 way, the cephalopoda approach the borders of the vertebrate 

 sub-kingdom. 



This remarkable class of animals affords in its details some 

 evidences in favour of the development theory. In the early 

 rocks of America, as in those of England, the humble form of 

 a straight or slightly curved shell prevails. Curved shells 

 increase afterwards. There are also tolerably distinct appear- 

 ances of a transition of forms in the genera of clymenia, gonia- 

 tite, and ceratite, which make their appearance in this succes- 

 sion in the rock formations. The dibranchiate belemnites 

 commence in the oolitic epoch, ushering in the sepias, the 

 highest of all the cephalopodous orders, and which have since 

 continued to exist. 



It is worthy of remark that, in the succession of rocks, the 

 forms of the cephalopoda change much more abruptly than 

 appears to be the case with other less organized mollusca ; 

 that is, there are more decided as well as more frequent 

 examples of what geologists call change of genera in this class 

 than in others. This is only one of the many proofs of law 

 in these phenomena. On the supposition of interferences, why 

 should there be entire renewals of some sets of animals and 

 not of others ? On the theory of law, we only see each line 



