112 STUDIES IN LIFE AND SENSE. 



ture and distribution in time ? In the answer to such a question, 

 asked concerning any group of living beings, lies the culminating 

 point of all biological science. That the cuttlefishes fall nominally 

 into their place in the scale of being indicated by evolution, and that 

 in their individual development, in the growth of their special organs 

 such as 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, cannot 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 specialised and lower form than the two-gilled 

 cuttlefishes, clearly points to the evolution of the two-gilled from the 

 four-gilled stock. The preponderance of the latter race in time, and 

 its long and solitary representation of the class, as well as the rela- 

 tively late appearance of the two-gilled species, are facts which 

 collectively point to the two-gilled forms as derivatives of the older 

 four-gilled race. The more active and structurally higher races of 

 to-day, in other words, have sprung from the less specialised and 

 lower cuttlefishes of the geological yesterday. No question, then, of 

 the reality of progressive development, as a factor in evolving new 

 species and groups of cuttlefishes from the confines of already formed 

 species, can be entertained. It is easy, moreover, to show from the 

 researches of Wiirtenberger that even in one group that of the am- 

 monites themselves the evidence of evolution is full and complete. 

 The Planulate or " ribbed " ammonites have, according to this author, 

 given origin to the Armata, or spinous forms. The ribs of the one 

 pass by gradual modification, well represented in the fossil shells, into 

 the spines of the other. So closely are these species of ammonites 

 connected that, as Wiirtenberger remarks, it is almost hopeless to 

 define where one species ends and another begins. The modifica- 

 tions of form which connect one curious shape of shell with another 

 of different shape and species are not sought in vain in palaeonto- 

 logical records. 



Turning more specifically to the shell in general, we may dis- 

 cover in the modifications of this single structure a clue to the entire 

 evolution of the cuttlefish race. The " shells" of the two-gilled 

 cuttlefishes 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 molluscs. In considering the nature of the various shells 

 of cephalopods, " palaeontology," as Professor Ray Lankester puts it, 

 "crosses the path of embryology." Starting with the shells which 

 are certainly oldest in point of time, and therefore of development, 

 we find in the Nautili and their neighbours, structures which repre- 

 sent fulness of shell-growth. It appears a long hypothetical journey 

 from the well- developed shell of the nautilus- type to the limy plate 



