DISTRIBUTION OF ORGANISMS. 161 



rent genetic line which actually appears, in every advance 

 of the animal kingdom, to be the case and developed con- 

 temporaneously with the weaker tribes, the fertility of which 

 would otherwise produce complete anarchy. Granting, then, 

 this pedigree for the cephalopoda, it would be no anomaly in 

 our theory, although remains of inferior mollusks should 

 never be found lower down in any part of the earth. 



The 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 appear- 

 ances 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 divi- 

 sion of the cephalopoda. In the whole of this order, however, 

 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. The humble 

 form of a straight or slightly curved shell prevails in the 

 earlier ages. Curved shells increase afterwards. There are 

 also tolerably distinct appearances of a transition of forms in 

 the genera of clymenia, goniatite, and ceratite, which make 

 their appearance in this succession in the rock formations. 



