EXTRINSIC CHARACTERS PROGRESSIVELY MODIFIED. 355 



expand in a definite direction in all the lines which assumed 

 it, but its rate of development in the different lines was not 

 uniform. 



The rapidity of development of this character may have 

 been determined, more or less, by environment, but the facts 

 seem to preclude the possibility of the determination of the 

 nature of the differentiation, or of the order of the sequence 

 of its expansion, by environment. We see here an exhibition 

 of evolution proceeding in a definite and continuous line of 

 expansion. It consists in a differential expansion in a defi- 

 nite direction and in a definite manner, by slow stages of 

 progress from generation to generation; and it is as distinctly 

 a predetermined law of evolution for the race as increase of 

 size and development of organs is a predetermined law for the 

 individual living organism at its birth. Environment checks 

 or accelerates it just the same as temperature or climate affects 

 the vigor of growth of the tree ; but the law of expansion 

 from Nautilian to Goniatitic, and then to Ammonitic suture 

 is the only one which the race can follow out ; and the ex- 

 pression of this law is as sure to follow in case the genera- 

 tions succeed each other, as the tree is sure to bear its appro- 

 priate fruit in case it lives and grows. 



Summary of the Laws of Evolution of the Suture-Lines of the 

 Ammonoidea. — The following may be given as a summary of 

 these interesting laws recognized in the history of the suture- 

 lines of the Cephalopod shells. The various suture-lines of 

 the chambered Cephalopod shells can be distinguished by the 

 differences in degree of complexity of the crimping of the 

 edge of the septum, viz. : 



{a) In the OrtJwccraii and Nautilian type the edge of tlie 

 septum is straight, or the curving is not enough to produce 

 more than a single oscillation of the suture-line during its 

 complete circumference. 



{b) The Goniatite septum presents a lobed suture, but the 

 edges of all the lobes and saddles are simple. 



(c) In the third type the lobes and saddles are variously 

 crenulated. In the Ccratite the crenulation affects the base 

 of the lobes, in Helictitcs the top of the saddles is crenulated, 

 and in Mcdlicottia the lobes, the saddles, and the connecting 

 parts of the suture are crenulated. 



