No. 5S0] ORIGIN OF SINGLE CHARACTERS 219 



characters and since they affect every bone in the skele- 

 ton we discover that characters of taxonomic import may 

 be found in every one of the small bones of the wrist and 

 ankle joints, which while less readily measurable are of 

 exactly the same kind of value in classification as the more 

 conspicuous changes of proportion in the skull and in the 



feet which Miller has used throughout in his definitions 

 of the Carnivora. In other words a "species" may as 

 consistently be defined by the proportion-characters of 

 one of its carpal bones as of one of its cranial bones; 

 such a definition would be strange and inconvenient, but 

 it would be quite as scientific. 



The rectigradations are also used in systematic defini- 

 tion only as soon as they become sufficiently large and con- 

 spicuous to be computed numerically. Looking up the 

 ancient definitions of the Eocene horses by Marsh and 



