CHAPTER XVIII 



ON SPECIES AND GENUS IN ZOOLOGY AND 

 PALAEONTOLOGY 



THE considerations just set forth on the series or 

 phyletic branches among fossil animals have al- 

 ready taught us some of the laws governing the 

 evolution of these branches ; and these laws are 

 the same for both Vertebrates and Invertebrates. 

 We have seen that this evolution is effected at a 

 variable rate of speed, but always by means of 

 a numerous series of branches, which develop on 

 parallel lines and by gradual mutations through the 

 different geological stages, without contact or tran- 

 sition from one branch to another, except in those 

 cases of bifurcation which we can very rarely grasp 

 with certainty. We have seen, also, that certain 

 groups, families, or genera are shown to be mono- 

 phyletic, and develop their mutations through one 

 series alone ; other genera, on the contrary and 

 these are the most numerous are polyphyletic, that 

 is, formed of a manifold sheaf of sub-branches, which 

 develop on parallel lines by following the same laws 

 as the principal branches. Finally, the course of 

 these branches is sometimes very long, and may even 

 cover almost the whole extent of geological times, 



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