CHAPTER XIV. 



DIGITS : RECAPITULATION. 



Ix the remarks preliminary to the evidence of digital Variation 

 it was stated that this group of facts is interesting rather as bearing 

 on morphological conceptions than from any more direct relation 

 to the problem of Species. The indications to be gained from the 

 evidence will be treated under the following heads: 



(1) Comparative frequency of digital Variation in different 



animals. 

 C2) Particular forms of digital Variation proper to particular 



animals. 



(3) Symmetry in digital Variation. 



i h The maims and pes as .systems of Minor Symmetry. 

 ( ."> ) Duplicity of limbs. 



(6) Homceotic Variation in terminal digits when a new 



member is added beyond them. 



(7) The absence of a strict distinction between duplicity of 



a given digit and other forms of addition to the 

 Series. 



(8) Discontinuity in digital Variation. 



(!)) Relation of the facts of digital Variation to the problem 

 of Species. 



(1) Comparative frequency of digital Variation in different animals. 



In reviewing much of the evidence of Variation and especially 

 in the evidence concerning the variations of teeth it has been seen 

 that the frequency of these variations is immensely greater in some 

 classes or species than in others. This is remarkably clear in the 

 case of the variations of digits. Compare for instance the great 

 frequency of polydactylism in the Horse with the complete absence 

 of recorded cases in the Ass. It is true that the latter is the rarer 

 animal, but it might still be expected that some record would have 

 liecn ioimd if the variation were as frequent in the Ass as in the 

 Horse. Again polydactyle Cats are certainly not very rare and 

 specimens are in several collections having been acquired at many 



