3l6 GEOLOGICAL BIOLOGY. 



identity there is under consideration but one species, though 

 it is constantly variable. The species exhibits constant plas- 

 ticity of several of its characters, but never reaches that fixa- 

 tion into separate forms which has been interpreted as the 

 result of the survival of the fittest by natural selection. 



Considerable and Continuous Plasticity of the Species. — The 

 width and form of the shell, the number of the striae, and the 

 concentric laminae constitute some of the more conspicuous 

 differentiae of the various forms of the species ; but, as David- 

 son says, " All these modifications can be traced in specimens 

 from any locality." 



In Murchison's " Siluria," (second edition,) is a remark 

 regarding the species, so pertinent that it is worthy of quota- 

 tion as it stands: "Among the Mollusca nearly all the 

 species of Atrypa, OrtJiis, and Spirifcr differ from those of 

 the Silurian age " (speaking here of Devonian Brachiopods). 

 " One shell, however, the Atrypa rctiailaris, must be men- 

 tioned as an exception to the prevalent rule of each great 

 group being distinguished by peculiar forms ; for this hardy 

 species, with which the reader became so familiar in the 

 Silurian rocks, lived on to the Devonian era, and is as com- 

 mon in the limestones and shales of Devonshire as in the 

 older rocks. It even ranges to the farthest known geographi- 

 cal limits of the Devonian rocks of Armenia, the Caucasus, 

 and China on the East, and to the Devonian deposits of 

 America on the West." 



Nature and Extent of the Variation. — The variations of this 

 species interested the acute naturalist Edward Forbes, and he 

 caused 117 specimens to be critically examined and the ribs 

 of each to be counted, and also the number of concentric 

 foliaceous expansions or fringes upon the surface. The 

 number of ribs, counting those on old and young specimens, 

 varied from ten to sixty, but there was found less divergence 

 in respect to the development and frequency of the concen- 

 tric fringes. Hisinger and Lindstrom, Davidson, Bronn, and 

 McCoy, among the earlier paleontologists, agreed in consid- 

 ering the forms with fewer and larger plications, called A, 

 aspera Schl. to be varieties of A. reticularis, but did not re- 

 gard them as distinct species. Lindstrom observed " that the 



