266 STUDIES IN THE EVOLUTION OF ANIMALS 



Raia, we have Nature's first tentative rough models of our own 

 hands and feet ! 



The present classification of vertebrates would be hardly 

 possible if all the extinct mammals, and all intermediate forms, 

 which have been either naturally selected out of existence or 

 destroyed by earthly catastrophes, were present before the mind 

 of the zoologist ; for the whole series would be more or less a con- 

 tinuous chain without the distinct walls of division or separation, 

 which sometimes form gaps in the present classification. 



Professor Agassiz ^ truly said : ' I have already stated that classi- 

 fication seems to me to rest upon too narrow a foundation when 

 it is chiefly based upon structure. Animals are linked together as 

 closely by their mode of development, by their relative standing in 

 their respective classes, by the order in which they have made their 

 appearance upon earth, by their geographical distribution,^ and 

 generally by their connection with the world in which they live, as 

 by their anatomy. All these relations should therefore be fully 

 expressed in a natural classification ; and though structure 

 furnishes the most direct indication of some of these relations, 

 always appreciable under every circumstance, other considerations 

 should not be neglected which may complete our insight into the 

 general plan of creation.' 



The upshot of all this discussion about the Horse's big digit is 

 to create a suspicion that it is possibly not an enlargement solely 

 of an ancestral third digit, but a fusion of the two digits of some 

 Ox-like ancestor. 



We know that the Ox— an animal seemingly much allied to the 



^ Contributions to the Natural History of the United States, quoted by Dr. C. F. 

 Holder in Life and Work of L. Agassiz, p. 183. 



^ Geographical distribution often unlinks animals. It is only by supposing some 

 means of translation that two distantly located animals of the same or similar species 

 can be considered as having had a common origin. 



