VARIATION IN TIME 163 



seas of the present day, where they are fished up 

 from a slight depth. If we compare one of the 

 earliest species of Lingulae, for instance, the Lingula 

 Lewisi of the Silurian of Gothland, with the living 

 Lingula anatina, we shall hardly note any other 

 modification than the greater size of the existing 

 shell, its rather narrower form, and its rather 

 more triangular top. It may thus be said that 

 the evolution of the phyletic branch of the 

 Lingulse has been almost null since the begin- 

 ning of Primary times. No doubt this remark- 

 able slowness of evolution bears relation to the 

 constancy of the conditions of the marine environ- 

 ment in which these animals have lived. 



Numerous examples of slow evolution as re- 

 markable as that of the Lingulae are known in the 

 history of palaeontology. The genera Lagena and 

 Rotalia, of the order of Foraminifera, extend from 

 the Silurian to our own day. The regular Urchins 

 of the Cidaris type are known from as early as the 

 Permian, and still exist in our tropical seas. The 

 Brachiopods of the genus Crania have lived, like 

 the Lingulse, from the lower Silurian to the present 

 time. In the Lamellibranch Molluscs, the living 

 genus Solenomya commenced in the Carboniferous 

 strata ; the present Nuculce and Ledas are found in 

 the series of strata from the Silurian onward ; in 

 the Jurassic some Pinnce are known, very slightly 

 different from living forms ; the Trigonice constitute 

 a numerous series of parallel branches in the Ju- 

 rassic and the Cretacean, and have persisted, 

 though much reduced in importance, down to our 

 existing tropical seas ; the interesting family of 



