Palm and Sole of Man.
The palm of man’s hand is a miracle of adaptations for touch and grasping, but has lost most of the coarse structure formed in response to stimuli of pressure and friction which we saw were common in lower mammals. This indeed he shares with most simian forms. The skin of our hands is now very much what we make it and responds very soon to fresh positive or passive conditions. The horny, cracked epidermis on palm and digit of the old sailor may be contrasted with the soft and flexible and pale surface of his twin-brother, the bank clerk, who is of studious habits and has neither the vice of gardening nor golf. If one compares the hand of the ordinary maid with that of her mistress the difference is striking. But if one compares the hand of that mistress with that of her spinster sister who has lain for twenty years in bed or on a couch, the difference is equally significant. Indeed the sofa-and-bed-ridden invalid, of whom I knew a few once, but who have gone out of fashion, gives the observer some useful thoughts as to the why and wherefore of the strange skin of the hands of the slow loris previously referred to. And if he be disposed also to the pleasant pursuit of moralizing at the expense of others he will feel led to reflect over harshly on the invalid and compare her outlook on life with that of the loris. Even in this concrete case of the hand of an invalid there may be evidence of positive as well as negative response, if one examines the right forefinger so much used in sewing, where the skin becomes hard and thick.
The foot of man has a good deal of negative evidence in favour of my contention as well as positive. As to the latter, in the thickening of the skin over the heel and ball of the great toe in those who walk much we find changes precisely similar to those on the hand. The negative or degenerative changes visible on man’s foot consist chiefly in the remarkable simplicity of pattern of the papillary ridges as well as their flattening and blurring, through wasting of those which occupy mainly the arch of the foot. These will be shown in the next chapter in a drawing. When this portion of skin is compared with that of the foot of any monkey or anthropoid ape it is clear that in this respect the skin of man’s foot has undergone even more degeneration than his hand has shown of higher development. This degeneration has coincided with two facts, first that man’s terrestrial locomotion has advanced far beyond that of any other Primate, and second, that he alone has a plantar arch. This subject belongs to a later chapter and is referred to here because the possession of an arch to his foot has caused man to escape, on the under surface of it, a vast proportion of the stimuli of pressure and friction involved in his mode of walking, and the extreme simplicity of his plantar papillary ridges, and relatively thin, soft skin under the plantar arches affords a fairly conclusive example of change of structure from disuse per se.
I have thus only selected and used two striking types of the Primates, the loris and man, not wishing to burden this part of the subject unduly with intervening and less characteristic forms of life. It may be legitimate here to say in defence of this long chapter that it illustrates what I desire to keep before me all through, the fact that use, habit, environment and selection go ever hand in hand. In all matters of science one has to descend to particulars, so it seemed necessary to select a few scattered phenomena in the best known groups of higher animals and endeavour to understand how certain “characters” or better “modifications” began to grow big enough to avoid passing through the meshes of the sieve.
CHAPTER XVIII.
ARRANGEMENT OF THE PAPILLARY RIDGES.
The subjects of the preceding, present, and the succeeding chapter are closely allied, from the fact that they all deal with structural changes in the mammalian skin, and that most of these are exhibited for us on our own palms and soles. They certainly comply with the canons of Henri Poincaré as to simplicity, regularity and chance of recurring.
In the last chapter, papillary ridges as organs of touch were briefly referred to, but their mode of development into complicated patterns do not concern the questions here at issue. The general manner in which they are arranged on the hands and feet of man and the Primates below him is very much a matter for such Lamarckian methods of inquiry as I have chosen. In this examination of the ridges I will proceed from man backwards among the Primates and lower still. I described these ridges, in a book previously referred to in the following words, and find no need to alter them here. “The ridges and adjoining furrows which cover the palmar and plantar surfaces of all Primates and a few lower forms in smaller degree, may be compared to the ridges of a ploughed field over which some object, as a light roller, has been passed, the effect of this being to produce a series of ridges with flattened tops. This can be well seen with a lens when the ridges are examined in profile, and is their normal condition in man and many lower animals, in nearly all the palmar, plantar and digital regions.”[64] The reservation in the last sentence is not material here.