98 THE DOCTRIXE OF DESCENT. 



that he has proved " that Perisphlnctes aurlgerus (Opp.) 

 of the Bathonians, and Perisphinctes curvirostrus of the 

 zone of the Cosmoceras Jason (Rein), are connected in 

 such a manner by intermediate occurrences that it is 

 impossible to draw a Hmit. 



L. Wurtenberger appHed his researches to thousands 

 of samples from the groups of the Planulate Ammonites 

 with ribbed shells, and of the Armate Ammonites with 

 prickly shells. In summing up his results he says, 

 among other things : " How among the Ammonites of 

 the Planulate and Armate groups, the species are to be 

 branched off from one another, I should be reluctant and 

 unable to give any instructions, for to me this question 

 appears utterly hopeless. For in groups of fossil organ- 

 isms, in which, as in the present case, so many connect- 

 ing links between the most extreme forms are actually 

 before us, that the transition is regularly carried on, the 

 species is far less susceptible of apprehension than in the 

 organic forms of the present world, which at least denote 

 the existing limits of the great pedigree of the organic 

 world. With respect to these fossil forms, it is funda- 

 mentally indifferent whether a very short, or a somewhat 

 longer portion of any branch be honoured by a special 

 name, and looked upon as a species. The prickly Am- 

 monites, classified under the name of Armata, are so 

 intrinsically connected, that it becomes an impossibility 

 to separate the accepted species sharply from one 

 another. The same observation applies also to the 

 group of which the manifold forms are distinguished 

 by their ribbed shells, and termed Planulata." It has 

 further transpired that the Armata, or Custata, originated 

 Irom the Planulata. 



