JAMES PERRIN SMITH 7 



In a genetic series of progressive forms all individuals in their 

 development should start out, theoretically, from the same stage, since 

 all must develop from an egg. Each individual would have to pass 

 through in its growth from the egg to maturity all the stages that the 

 successive generations of mature forms passed through during the long 

 history of the race. Characters that were present at maturity in the 

 ancestors should appear by palingenesis in the development history of 

 the descendants, and the coenogenetic, or later characters, should grad- 

 ually be pushed back into the ontogeny. 



In a general way, too, this is true. As, for instance, in the 

 Ammonoid stock the primitive simple shell, with its calcareous proto- 

 conch and siphuncle, when once introduced as a coenogenetic or secondary 

 character, persists throughout the history of the race, becoming a 

 primary character, and finally appearing only as a palingenetic character 

 in some of the modern cephalopods. All this is seen in the history of 

 the race from the primitive Orthoceras of the early Paleozoic, with its 

 chambered shell and siphuncle, but without the calcareous protoconch 

 or embryonic shell. Some members of the Orthoceras group finally 

 acquired a calcareous protoconch, and this soon introduced with it 

 another cffinogenetic character, the marginal position of the siphuncle, 

 forming the group of Bactrites (PI. XIV, fig. 7), which was to become the 

 starting point for the Ammonoids and the Belemnoids. Some Bactrites 

 began to become coiled, and developed into the primitive Goniatites, 

 {Mimoceras, PI. XIV, fig. 8). Others remained straight, but began to 

 cover up the slender shell with the mantle, and finally to secrete a sec- 

 ondary covering of lime to protect it, growing into the race of Belemnites. 

 But even in the Belemnites the chambered shell, inherited from the parent 

 Orthoceras is still retained as a youthful character, once ccenogenetic, but 

 now so long present in the race history that it is pushed back into the 

 larval stages, and finally appears as a mere reminiscence only in the em- 

 bryology of some sepioids. 



The coenogenetic lime secretion that covered the chambered shell of 

 the Belemnites has had a similar history, disappearing in most modern 

 forms, but retained as a vestigial character in the cuttlefish "bone."' 



All characters were once secondary or coenogenetic, and all may be- 

 come primary, and finally vestigial. 



