THE FORE FEET OF MAMMALS, 35 
be more surprising if the fore feet of the dog also (Fig. 4), 
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 bat (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, 
with 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. 
