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Updated: May 11, 2025
In the organic world the perpetual modification of structures that has been effected through natural selection exhibits countless instances of differences in kind which have risen from the accumulation of differences in degree. No one would hesitate to call a horse's hoof different in kind from a cat's paw; and yet the horse's lower leg and hoof are undoubtedly developed from a five-toed paw.
But in all these cases we can prove that some of the toes atrophied, and were in time lost altogether. The causes of this evolution of the five-toed foot from the many-toed fin in the amphibian ancestor must be sought in adaptation to the entire change of function that the limbs experienced in passing from an exclusively aquatic to a partly terrestrial life.
We leave camp at 8:20. I put out fire while men are packing. Find track of small five-toed animal on the trail. We go by cattle-trails a short cut to Swamp Point through the forest, over ridges, through thickets and some of the grassy valleys. Out on Swamp Point again I am shown Bass Camp on the south rim. It is scarcely discernible even with glasses, the distance is so vast.
Fresh turkey tracks showed in places, and on the top of the longer dam, fresh made in the mud, were lion tracks as large as the crown of my hat. How sight of them made me tingle all over! Here was absolute proof of the prowling of one of the great cats. Beaver tracks were everywhere. They were rather singular looking tracks, the front feet being five-toed, and the back three-toed, and webbed.
The first indication apart from certain disputed impressions in the Devonian of the land-vertebrate is the footprint of an Amphibian on an early Carboniferous mud-flat. Hardened by the sun, and then covered with a fresh deposit when it sank beneath the waters, it remains to-day to witness the arrival of the five-toed quadruped who was to rule the earth.
No such five-toed "atavus," however, has yet made its appearance among the few middle and older Eocene Mammalia which are known.
In their whole organisation even the actual Amphibia seem to be an instructive transitional group. In the important respects of respiration and circulation they approach very closely to the Dipneusta, though in other respects they are far superior to them. This is particularly true of the development of their limbs or extremities. In them we find these for the first time as five-toed feet.
No such five-toed "atavus," however, has yet made its appearance among the few middle and older Eocene Mammalia which are known.
These children grew up, and when they came to adult years, they all married, and of course it happened that they all married five-fingered and five-toed persons. Now let us see what were the results.
And the knowledge we now possess justifies us completely in the anticipation, that when the still lower Eocene deposits, and those which belong to the Cretaceous epoch, have yielded up their remains of ancestral equine animals, we shall find, first, a form with four complete toes and a rudiment of the innermost or first digit in front, with probably, a rudiment of the fifth digit in the hind foot; while, in still older forms, the series of the digits will be more and more complete, until we come to the five-toed animals, in which, if the doctrine of evolution is well founded, the whole series must have taken its origin.
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