THE FORE FEET OF MAMMALS. 35 



be more surprising if the fore feet of the dog also (Fig. -t), 

 as well as the breast-fin (the hand) of the seal (Fig. 5), and 

 of the dolphin (Fig. 6), show essentially the same structure. 

 And it will appear still more wonderful that even the wing 

 of the hat (Fig. 7), the shovel-feet of the mole (Fig. 8), and 

 the fore feet of the duck-bill (Ornithorhynchus) (Fig. 9), the 

 most imperfect of all mammals, is composed of entirely 

 the same bones, only their size and form being variously 

 changed. Their number, the manner of their arrangement 

 and connection has remained the same. (Compare also the 

 explanation of Plate IV., in the Appendix.) It is quite incon- 

 ceivable that any other cause, except the common inheritance 

 of the part in question from common ancestors, could have 

 occasioned this wonderful homology or similarity in the 

 essential inner structure with such different external forms. 

 Now, if we go down further in the system below the mam- 

 mals, and find that even the wings of birds, the fore feet of 

 reptiles and amphibious animals, are composed of essentially 

 the same bones as the arms of man and the fore legs of 

 the other mammals, we can, from this circumstance alone, 

 Avith perfect certainty, infer the common origin of all these 

 vertebrate animals. Here, as in all other cases, the degree 

 of the internal agreement in the form discloses to us the 

 degree of blood relationship. 



