One may observe that there’s a divinity doth shape our ends, rough hew them as we may, even if some objection be taken to the present view of rough-hewing of parts of our organism on the ground of its piecemeal character, rather than dealing with the organism as a whole. To which it may be replied that the Mendelians give high support to the piecemeal study of the profound subject of genetics, and further that the business here is to look separately and simply at a few selected attributes of parts of an organism, and see how they began to grow big enough to avoid passing through the meshes of the sieve.
The foregoing examples of animals in which papillary ridges are absent have been given not in their zoological order, nor as representative of a great many groups, but as taken from the eighty-six species I examined myself. The following belong to the same series, but all present papillary ridges in an ascending scale towards perfection in man.
Examples of Ridge-covered Palms and Soles.
The common hedgehog though a burrowing animal like the mole is not always underground as his distant relative is. He is not always mining and though of ancient lineage he is a “slacker” compared with the mole, hibernating for months, and spending also much time in his nest and prowling slowly about above ground for insects. He has thus acquired his somewhat indifferent epidermis that one finds, but with the addition of sparse papillary ridges. It is the species among this list with the fewest of these tactile structures, for there are but three or four separate ridges on six of the ten digits, and radiating groups on only three of all the palmar and plantar pads. So quâ touch it is ill-equipped, though it has adapted a higher form of tool than the rabbit.
The common squirrel, that sits much and walks mainly on branches of trees just as much as it needs to do, has an epidermis little differentiated, and one which is corrugated with scanty papillary ridges on the palmar and plantar pads, and none on the digits.
The squirrel-like phalanger which flies always more or less downwards by a kind of parachute-arrangement has most of its palmar and plantar skin covered with papillary ridges encroaching upon its corrugated areas, and a response to more delicate tactile experience has been thus produced by its intermittent performance of ordinary progression.
Azara’s opossum presents about as large a part of the surface covered with nodules as with papillary ridges, the latter highly-developed for an animal so low, zoologically-speaking, but one in which delicate discrimination is much practised.
The kinkajou, another arboreal animal which walks about on trees more than it uses its feet for prehension, trusting much to its prehensile tail, shows its corrugated epidermis and papillary ridges developed in about equal proportions.
These five mammals thus show that the stimuli of pressure and friction and the response to them are being complicated by the addition of the more delicate tactile organs known as papillary ridges, and these, perhaps, in a secondary way are becoming useful in preventing friction. But I must not omit to point out that, quâ prevention of slipping, the few sparse papillary ridges of the hedgehog, squirrel, kinkajou and flying-phalanger, especially those on the extreme tips of the digits, could have no effect in this prevention and no survival value. It is otherwise when they are developed in large areas as in the succeeding groups.