THE ONE BIG DIGIT OF THE HORSE 249 



same plan. I think the dogma that our hand is made up of five 

 digits only, like that of Phoenacodus, is not so unquestionable as 

 might at first appear. 



Modifications have been so vast, from the first appearance of a 

 hand and foot, in vertebrates, up to the form with five digits, that 

 it is next to impossible always to discover, in the whole series, 

 which bones are homologous. 



In the changes that the carpal and tarsal bones have been 

 subjected to, it is not easy to identify their position regarding the 

 metacarpal and metatarsal bones. For instance, in the extinct 

 Coryphon and the Titanothere} there is already a displacement of 

 wrist and ankle bones. If in these primitive animals the change 

 of position is already considerable, what shall we say of their posi- 

 tion in the Horse and Ox? and in the tarsal bones of Man the 

 change of position is vast. 



In the higher mammals, where these limbs have not been much 

 modified in structure, of course the homologies of bones may 

 be easily ascertained. But even here it is not always easy to 

 do so. 



Flower and Lydekker ^ give the right hand of a water Tortoise 

 [Chelydra Serpentina), Fig. 94. It is an animal far removed from 

 mammals, but nevertheless it has five digits in its hand, and is 

 evidently related to the human hand. It has a separate carpal 

 bone to each digit, all in one row. This order becomes much 

 modified in the carpus of other animals, so much so that Chauveau * 

 says : ' The human hand having five digits and five metacarpal 

 bones, it is rational to admit the virtual existence of five pieces to 

 each of the carpal rows. Materially, there are only four bones in 

 each of the rows ; but the comparative study of the relations of 



1 Royal Natural History, p. 152. ^ Mammals, p. 48. 



' Anatom. Comp. des Animaux domestiques , tr. by Fleming, p. 123. 



