162 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



descendants of the typical Oxynoticerata of the 

 lower Jurassic. 



Without multiplying further these examples of 

 discontinuous or intermittent series, it will be seen 

 what special difficulties these gaps cause in the 

 certain reconstitution of phyletic branches. It 

 may, however, be hoped that these difficulties will 

 disappear, one by one, as more complete geological 

 researches enable us to discover the intermediate 

 links which for the moment are unknown. 



VARIABLE RATE OF THE EVOLUTION OF 

 BRANCHES. Nothing seems to have been more 

 variable than the rapidity of the comparative 

 evolution of the different phyletic branches. In a 

 certain number of them the chronological modifi- 

 cations are insignificant or almost null during 

 nearly the whole duration of the geological periods. 

 An often quoted example is that of the genus Lingula, 

 belonging to the group of inarticulated Brachiopods 

 characterized by a thin, chitinous shell, with equal 

 valves, subrect angular in form, with a somewhat 

 prolongated top, and fixed to submarine bodies by 

 a long, flexible pedicule. The Lingulce figure among 

 the earliest organisms known in the Primary seas. 

 Without taking into account the Lingulella of the 

 Cambrian, which would appear to constitute a 

 small independent branch, the true Lingulce first 

 appear in the Silurian, where they comprise, 

 according to Bigsby, about a hundred species. 

 They are already diminished in numbers in the 

 Devonian and the Carboniferous strata, and still 

 more so in Secondary and Tertiary times. But the 

 branch none the less persists down to the tropical 



