Page:Popular Science Monthly Volume 21.djvu/784

766 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 specialized 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 development, 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 evolution 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 mollusks. 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 appears 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 regarded as the degenerate remains of structures fully developed in their ancestors. To this idea, their succession in time bears faithful witness; 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 seabeds of the far-back Mesozoic, and still more remote Palæozoic 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,