PLASTICITY AND PERMANENCY OF CHARACTERS. 305 



responding formations), in each of the cliief types, great fluc- 

 tuation in this character; i-io, perhaps, is not too much. In 

 later periods there is variabiHty, but each species is subject 

 to less variation, so that mathematically the species might 

 be said to have this character variable in separate cases, 1-3, 

 2-5, '})-'], 5-9, etc. ; and there are certain lines of forms in 

 which the general range of this variability continues the same 

 from period to period. 



As to the size of the fissure in proportion to the other 

 parts of the shell, there is considerable variation, but it is 

 probably co-ordinate with the development of the area, those 

 with high area having narrow fissure, those with low area a 

 broad fissure. The characters, therefore, of the delthyrium 

 and its cover show, in respect of evolution, purely extrinsic 

 modification, the characters reaching extreme range at first, 

 and afterwards, in the various races, expressing modification 

 by restriction of variation and adaptation to special or local 

 conditions. 



D. Hinge Ai'ca. — This may be very narrow and elongated, 

 forming a long hinge-line, or it may be very high, forming a 

 triangular and greatly developed area and ventral beak. I 

 know of no species, or sets of forms, which express a greater 

 range of modification of this feature than the two species 

 called Spirifcr plicatclla and Cyrtia cxporrccta of the Wenlock 

 limestone. The specimens with high beak are generally 

 called Cyrtia; the specimens with moderate or low beaks are 

 Spirifer. This character ranges from i — 10 in the earliest 

 stage. In other species {S. cj'ispiis and its associates) there is 

 a less degree of modification of this character (Figs. 88—91). 

 In later forms the range of modification for each species is 

 generally confined within less limits. The extreme extent of 

 the modification and the extreme forms themselves are gen- 

 erally met with where the species are most abundant, and the 

 prevalence of one extreme or the other is expressed in the 

 later end of a series, which from the close resemblance of the 

 successive specimens constituting it may be considered to be 

 a true genetic series or race. Here again we find evidence 

 that whatever evolution takes place is extrinsic and results, 

 theoretically, from adjustment to environment, selection in 



