PLASTICITY AND PERMANENCY OF CHARACTERS. 311 



cision in a series of specimens of Spirifer mesocostalis Hall, 

 which, in the Middle Devonian, shows in most specimens no 

 trace of a median septum in the ventral valve, occasionally a 

 variety appearing with a mere line representing the septum. 

 At the base of the Upper Devonian (Ithaca group) frequent 

 specimens with slight development of the septum are seen ; a 

 little higher, in the middle and upper part of the Upper 

 Devonian, the septum is conspicuous and is strongly de- 

 veloped. In other species the development of septal parts 

 appears to be varietal ; the older shells, in general, express- 

 ing fuller calcification of parts which are supports or partitions 

 between active organs of the animal. Among the Spiriferidae 

 there are several such lines of species, as the Cyrtina and the 

 Spiriferina; and in fact the forms which are punctate are all 

 more or less prone to develop calcified supports or partition 

 plates. 



Evolution of Extrinsic Specific Characters Comparatively Slow, 

 although their Plasticity is Greater at the Initial Stage, In all 

 of these characters, which constitute the specific differentiae 

 of the species concerned, we observe a relative slowness of 

 evolution which is quite consistent with the laws of natural 

 selection, of gradual acceleration or retardation by hereditary 

 means, and of the perpetuation of favorable characters by the 

 dropping out of others ; but at the same time we notice at 

 the early stage of the life-history of the genus, or subfamily, 

 a marked plasticity in respect of most of these characters- 

 which is in strong contrast with the fixity and persistence, 

 without change, of the characters of higher rank which mark 

 the family, and appear to have arisen at the same time. 



Laws of Intrinsic and Extrinsic Evolution expressed in Varia- 

 bility and Permanency of Characters. Among the first repre- 

 sentatives of the family there are family characters which 

 are repeated thereafter in numerous individuals for several 

 periods of geologic time without noticeable change, and they 

 did not appear before. There are also characters appearing 

 on the first species which vary and show slight change all the 

 way along thereafter, and are themselves less different from 

 the characters of previous forms : relatively, one set of char- 

 acters appears and thereafter a long line of successors follow 



