

VARIATION IN TIME 157 



regarding the researches of Neumayr on this fine 

 phyletic series. 



THE POLYPHYLETIC GENERA. But this learned 

 palaeontologist has taken us still further into the 

 natural processes of realization of these phyletic 

 series. He has shown us, in fact, in his brilliant 

 study on the Ammonites of the genus Phylloceras, 

 that each great genus, however homogeneous it 

 may appear, does not evolve in one single line, 

 but really comprises a series of parallel branches 

 of simultaneous evolution of unequal duration. 

 We are here confronted by a truly general law of 

 the highest importance, which seems to justify 

 a rapid analysis of the evolution of the genus 

 Phylloceras so well studied by Neumayr. 



The Phyllocerata are closely coiled shells, the 

 last whorl of which completely envelops the inner 

 ones. It has a smooth surface, or one very slightly 

 ornamented with fine stripes, excrescences, or 

 transverse furrows. This genus, which is very 

 homogeneous, if we consider its numerous species 

 as a whole, is especially characterized by the leaf- 

 like termination of the prominent parts of its 

 sutures, whence the generic name given to it by 

 Ed. Suess. The family of which 4)his genus forms 

 part is known as early as the Trias ; but the Phyllo- 

 cerata properly so-called only commence at the 

 infra-Lias sic epoch, and continue as far as the Upper 

 Chalk. Neumayr recognized among the Jurassic 

 and Cretacean forms five parallel phyletic series 

 which he describes as follows : 

 ^fel. The Phylloceras heterophyllum series, with 

 either a smooth shell or one ornamented with fine 



