98 THE DOCTRINE OF DESCENT, 
that he has proved “ that Perisphinctes aurigerus (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 limit. 
L. Wiirtenberger applied 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 
from the Planulata, 
