To the invaluable records of creation furnished by palæontology and ontogeny are added the no less important evidences for the blood relationship of organisms furnished by _comparative anatomy_. When organisms, externally very different, nearly agree in their internal structure, one may with certainty conclude that the agreement has its foundation in Inheritance, the dissimilarity its foundation in Adaptation. Compare, for example, the hands and fore paws of the nine different animals which are represented on Plate IV., in which the bony skeleton in the interior of the hand and of the five fingers is visible. Everywhere we find, though the external forms are most different, the same bones, and among them the same number, position, and connection. It will perhaps appear very natural that the hand of _man_ (Fig. 1) differs very little from that of the _gorilla_ (Fig. 2) and of the _orang-outang_ (Fig. 3), his nearest relations. But it will 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 inconceivable 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 mammals, 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. CHAPTER XVI. PEDIGREE AND HISTORY OF THE KINGDOM OF THE PROTISTA.