7 66 THE POPULAR SCIENCE MONTHLY. 



eye and ear, as well as in the general relations they bear to each other 

 as living forms, they illustrate the results of progressive development, 

 can not for a moment be doubted. The further fact that the existing 

 four-gilled nautilus, despite its lengthy ancestry, as regards its brain, 

 its eye, its tentacles, and other features of its history, is a less special- 

 ized and lower form than the two-gilled cuttle-fishes, clearly points to 

 the evolution of the two-gilled from the four-gilled stock. The more 

 active and structurally higher races of to-day, in other words, have 

 sprung from the less specialized and lower cuttle-fishes of the geological 

 yesterday. No question, then, of the reality of progressive develop- 

 ment, as a factor in evolving new species and groups of cuttle-fishes 

 from the confines of already formed species, can be entertained. 



Turning more specifically to the shell in general, we may discover 

 in the modifications of this single structure a clew to the entire evolu- 

 tion of the cuttle-fish race. The " shells " of the two-gilled cuttle-fishes 

 exist for the most part as horny " pens " or as limy plates, secreted by 

 the " shell-gland " of the mantle which forms the true shell of all mol- 

 lusks. Starting with the shells which are certainly oldest in point of 

 time, and therefore of development, we find, in the Nautili and their 

 neighbors, structures which represent fullness of shell-growth. It ap- 

 pears a long hypothetical journey from the well-developed shell of the 

 nautilus type to the limy plate or horny " pen " shell of the squid. 

 But the halting-places on the way diminish the apparent length of the 

 journey, as they lessen the seeming irregularity of the path. The 

 simple rudimentary shells of our two-gilled cuttle-fishes are to be re- 

 garded as the degenerate remains of structures fully developed in their 

 ancestors. To this idea, their succession in time bears faithful wit- 

 ness ; and to its correctness the connecting links, accessible to us, 

 plainly testify. 



Thus the history of the cuttle-fish shell forms an important chapter 

 in the biography of the race. The rudimental shells of the two-gilled 

 cuttle-fishes, like the teeth which never cut the gum in unborn whales, 

 have a reference not to their present life, but to a former state of 

 things. Contemplating the " pen " or " cuttle-bone " of a modern 

 squid or sepia, our thoughts become molded in mental continuity with 

 the past. There rise to view before our mind's eye the ancient Nautili 

 and their sculptured kith and kin the ammonites, crowding the sea- 

 beds of the far-back Mesozoic, and still more remote Palaeozoic ages. 

 Then, through the operation of the inevitable laws of organic progress 

 and advance making the ancient world then, as they constitute our 

 world to-day, the theatre of continual change we see the two-gilled 

 stock arise in secondary times from the four-gilled race. First there 

 is seen the modification of shell. Concurrently with the decrease of 

 shell comes increase of head-development and elaboration of nerve- 

 centers, tending to make the new two-gilled form what we know it to 

 be to-day the wary, watchful organism, living in the waters above, 



