Some Chosen Examples of Palms and Soles.
The facts then of a few selected examples of the palms and soles of mammals are shortly these.
A heavy, burrowing animal, the earth wolf of the Cape, has a very smooth, hard epidermis covering its foot-pads and is thus a generalised structure which I have found in no other animal.
The common mole which uses its broad strong fore-feet like a pair of spades, and depends chiefly for discrimination of its habitat on the delicate sensory nerve-endings of its snout, has a hard nodular skin which is much less developed on the hind feet than the fore feet, the latter being less active tools. It has no papillary ridges, in accordance with this fact, and is a very efficient miner that never practises ca’ canny, as we know to our cost when we go out in the morning and find great heaps of soft earth thrown up in the line of its advance from its base or fortress. Such a mode of life lends itself remarkably to the kind of skin on its feet, and this is now at any rate adapted to its environment.
The capybara is a large, heavily-built rodent, and has rather a smooth epidermis not specially thick, with long and efficient papillæ of the corium shown in microscopical sections. Being largely aquatic in its habitat, and given to frequenting marshy ground and to enjoying as much sleep as it can manage, it depends a good deal for discrimination of objects on its sensitive corium, and its epidermis is not much specialised for, or by friction and pressure in walking. It does not acquire by reason of stimuli and response any unnecessary tools.
With this may be classed the echidna or Australian ant-eater which has sparse hairs set on a hard and slightly corrugated epidermis, and, being mainly a nocturnal animal and living a secluded life, it does not walk much or far in its stealthy pursuit of worms and insects, and the stimuli of friction or pressure encountered by it are few.
A similar condition is found on the feet of many small carnivores.
Animals with scales on their feet, which are held to constitute the earliest stage of the Primate modification of papillary ridges are such as the potoroo, wallaby, kangaroo and giant ant-eater. Such scales register a long, long series of stimuli of friction and pressure in these and their ancestors, in a level of life before any delicate discrimination of surfaces came into operation.
The nodular form of skin is present in the Canadian tree porcupine, where rough nodules cluster closely on the surface of both feet, and it is a significant fact that it shares with the American opossum the peculiarity of nodules on the ventral surface of the powerful prehensile tail. This adaptation tends to efficiency in its arboreal life, and may well have been produced by infinitely small degrees of response in structure in the course of a long evolution.
The rabbit alone have I found with rod-like projections of the epidermic cells, among which are set in dense order the soft, long, delicate hairs and which thus conduce to its wonderful power of treading on sharp objects without injury. We thus see the inner meaning of dear old Brer Rabbit’s jeer of triumph to Brer Fox, “Born and bred in a brier bush.” This adaptation might be an unit-character segregated from the ancestral stock of the Leporidæ, or it might not, but at any rate the rabbit leads a life in which its walking or running is no more prominent or frequent than is a good “run” on the part of a hunter which pursues the hare with his beagles, and one may say at least this—that its mode of life has not produced a hard rough nodular surface on its feet by stimuli of pressure and friction and response.