PLASTICITY AND PERMANENCY OF CHARACTERS. 315 



the same time to the fact that these systems, which have been 

 recognized as well-cstabhshed natural divisions in the geologi- 

 cal scale of formations throughout the northern hemisphere, 

 were also distinguished at their beginnings by marked change 

 in the life of the world. Not only do new types of genera 

 and families appear, but even the specific types of a continu- 

 ous race of species express the changes incident to the open- 

 ing of a new period. 



Whatever be the explanation, these facts make it evident 

 that the divergence of a generic type into different subgeneric 

 expressions was not by slow and accumulative process, but 

 by relatively rapid expansions, followed by the continuance of 

 the types with gradual but restricted modification until the 

 race died out. The divergence of these types from each 

 other was very early in the history of each race, and in the 

 present case there was evidently a secondary divergence at 

 the beginning of the Devonian, and a slight tertiary diver- 

 gence, in the Aperturati group, at the beginning of the Car- 

 boniferous. 



Characteristics of the Life-history of Atrypa reticularis. — 

 Atrypa reticularis may be taken as an example of a species 

 which exhibits scarcely 

 any trace of what has 

 been called extrinsic evolu- 

 tion, but has lived a long 

 time, been very fertile, 

 has been distributed all 

 around the world, and has 

 shown its adaptability to a 

 great variety of environ- 

 mental conditions, without '?\G.<i'&.— a, B, Atrypa reticularis XAnn./ ={0x2.- 



men ; cr — crura ; b = jugum ; sp = spiral coils of 

 Sufferinrr any appreciable thebrachidium.^adult natural size- S, young 



" -^ '^ '^ specimen, magnified one hfth. (r rom bteinraann 



morphological change. It and Doederlein.) 



began with the initiation of the genus, and lived throughout the 

 life-period of the genus, which is almost equal to that of the 

 family or suborder to which it belongs. The species has re- 

 ceived a great many names, and been referred to many genera ; 

 but the more careful the study applied to it, the more clearly 

 does it appear that under all proper discrimination of specific 



