77/0 Life-Histoi-ji of Spiri/vr hen's. 150 



centric lines of growtli at the lateral extremities of the shell ; 

 with the ohlitcration of the longitudinal plications, there is a co- 

 ordinate expansion of the front and lateral margin, causing a 

 relative shortening of the cardinal margin and a shorter bending 

 of the concentric striae to meet it at the extreraitiesi, — and at the 

 same time an increased growth upwards of the hinge-area. So 

 that we find high area, — short hinge-line, — abrupt curving of 

 the lines of growth at the cardinal extremities, — and tendency 

 to the obliteration of radiate plications, — to be co-ordinate fea- 

 tures of the typical form whose history we are here studying. 



By examination of other Spirifers, we discover great variety 

 of shajDC, due to variation of hinge-extension and elevation of 

 area, with sufficient constancy of other characters to constitute 

 good species : — for instance, *S'. mucronatns, S. medialia and 

 ;S'. disjunctiii>, and the allied forms to which each may be sup- 

 ])Osed to stand in the relation of types. A comparison of the 

 varieties of each suggests that the typical form of S. mucrojia- 

 his has a widely extended hinge, low area, and produced ex- 

 tremities; — that the type of S. medialis has a shorter hinge-line, 

 not produced into a point, with moderately high area. 



A I'efcrence of the Portage Spivifer loevis directly to an origin 

 in the S. fimhricdus of the preceding period, seems to need no 

 argument further than the presentation of the facts, and a com- 

 parison of it with the various forms, earlier and later, with 

 which it is most closely related. But a deeper study of the 

 facts leads us to an equally clear conclusion that ^S'. Jimbriatus 

 is only a variety of still earlier forms, and that the characters 

 marking each variety appear as variational forms of the early 

 type, and that during the passage from one to the other no 

 assumption of new characters has taken place, — such as would 

 not be regarded as purely varietal among living organisms, 

 consisting in the obliteration or obscuring of prominent charac- 

 ters in some of the later rejDresentatives. An examination of 

 Carboniferous forms shows the continuation of each of the typi- 

 cal characters in some representatives of the or'ginal stock. 



There is no evidence of crossing of breeds to pi'oduce new 

 varieties, — but merely a localising and interbreeding of varieties, 

 to the production of greater prominence and fixity of certain 

 characters. 



