Meaning.

Whatever be the meaning and origin of these flexures they are not mere folds such as one makes in a garment and leaves it so. Action, function and fitting of the structures of the hand and foot are involved in their history. They may loosely be termed “ergographs” without any reference to the exact measurement of work done. No proper idea can be formed of them if the original function and evolution of the walking-pads of earlier mammals be omitted. If one goes back and back until one reaches some lowly marsupial as a vulpine phalanger, or insectivore such as a common hedgehog, one may even metaphorically see these animals being fitted by a shoemaker with rude shoes or walking-pads for the better locomo­tion on or under ground, or in the branches of trees. These pads are projecting masses of hard fat with fibrous tissue interspersed and they early become fitted or adapted to or by the use to which they are put. It is impossible to suppose that certain rudimentary pads are devised by selective processes prior to the altered habits of walking of the animal that acquires them. From the shoemaking point of view the fashion is rough and generalised, and the changing habits of the animal adapt the shoe by degrees to the function employed, much as many a private soldier knows to his cost that he has had to adapt slowly and painfully his army boot to his particular foot. This process in an early pedestrian mammal involves the breaking up and limiting of the rudimentary pads by sulci in the dense skin, and the process of struggle and adjustment between the pads and their bordering furrows issues in the characteristic flexure of each mammal. From experiences in the human body one knows how easily fibrous adhesions between the skin and deeper parts, notably in cases of Dupuytren’s contrac­tion of the palmar fascia, are formed by close apposi­tion of the two layers. Such adhesion is precluded when much movement of the part occurs, but ex-hypothesi the rudimentary flexures are distinguished by absence of movement, and the conditions for fixing down the deeper layers of the skin to the bones beneath are clearly present. That these are not indifferent structures is evident from what Macalister says, and though they be small or even trivial may be held to have acquired at some time or other selective value. Their early stages would necessarily be too tentative, varied and slight to acquire such value.

Fig. 72.—Foot of common squirrel.

Fig. 73.—Flexures on foot of vulpine phalanger.

Fig. 72 is a sketch of the hand and foot of a squirrel (Sciurus) and the numbers 1, 2, 3, 4, 5 and 6 are placed conspicuously on the walking pads in accordance with the teaching of Dr. and Mrs. Wilder Harris as to the six palmar and plantar walking-pads, of which the typical palm and sole is constructed. The thick, black lines indicate the flexures formed round the pads by the exercise of the functions of the hand and foot.

Fig. 73 represents the clumsy, thick walking-pads of a marsupial the vulpine phalanger, trichosurus vulpecula.

Fig. 74.—Foot of loris.

Fig. 75.—Foot of ring-tailed lemur.

Fig. 74, the highly-developed prehensile foot of the loris.

Fig. 75, the foot of a ring-tailed lemur.

Fig. 76.—Foot of squirrel-monkey.

Fig. 77.—Foot of macaque.

Fig. 76, the foot of a squirrel-monkey (Chrysothrix Sciurea).

Fig. 77, the foot of a macacus (Macacus cynomologus).

Fig. 78.—Foot of gibbon.

Fig. 79.—Hand of chimpanzee.

Fig. 78, the foot of a gibbon.

Fig. 79, the hand of a chimpanzee and here the resemblance to the hand of man and not to the foot of man is very striking.

A description has already been given of man’s flexures of the palm.

Fig. 80.—Drawing of flex­ures of sole of foot in young adult.

Fig. 80 is a careful drawing of the sole of a young active woman with a well-formed foot, and there is little typical in the mode of arrangement of its creases except the slight tendency to transverse lines of flexure. In all the feet I have examined I have found no single flexure that is constant, and the longitudinal ones here shown are often absent.

Reviewing these examples one observes an evolutional decay of a minor but necessary piece of mechanism of the Primate hand and foot. The general similarity, mutatis mutandis, of the flexures of the palm and sole in Primates is very noticeable, and is associated with the strong prehensile power of the foot of all the forms below man. In the cases of the two apes shown in this series, the resemblance is still well marked, more so even in the chimpanzee than the gibbon, so that the disappearance from the sole of man’s foot of any important flexure is very significant of his loss of prehensile and gain of locomotive perfec­tion, and I find it impossible to conceive any process of evolutionary change where a loss of the flexures of a prehensile foot could come under the power of selection, on its own merits. On the other hand this remarkable instance of disuse of a formerly useful structure is adequately accounted for by the evolution of an organ like the human foot which in course of long periods of time became an organ of one function. Weismann might score a point over Spencer from his laboured explanations of man’s dwindling little toe, but here, I submit, he would have had to take refuge in silence, and pass to characters of a higher and more debateable kind.

CHAPTER XX.
THE EVOLUTION OF A BURSA.

A bursa exercises a function in the animal body which is the direct opposite of that shown to belong to the flexures of the hand and foot. Whereas the latter are adapted to the preven­tion of slipping in the act of prehension, bursæ are delicate contrivances for producing the maximum effect of sliding, within certain limits, between two opposed surfaces, either between the skin and a hard surface beneath it, between two muscles, or a tendon as it moves over a bone. As they are very variable and most of them are inherited and congenital, while some are produced only in the lifetime of the individual, they are useful for considera­tion in regard to the questions of transmission of modifications and of the origin of initial variations. Their degree of utility ranges, for example, in man, from that of the prepatellar bursa without which no useful movement of the knee-joint is imaginable, to the insignificant bursa which may or may not be found on the dorsal surface of a phalangeal joint of the foot. The principle laid down by Lyell, to which allusion has been made elsewhere, that is, of “explaining changes in the surface of the earth by reference to causes now in action,” is applicable in this small department of the evolution of a minor structure of the animal body. As man furnishes the largest of all collections of these lubricating organs, his skeleton and skeletal muscles will form the main subject of this chapter, and I venture, if one may say so, to “Lyell” them. None of the sections of this book except that on the mammalian hair affords so simple and easy a field for watching in operation certain mechanical forces. We may here go down to the potter’s house and watch him moulding his clay, or the cobbler his leather. So much are bursæ in the human body under the power of extraneous forces that I venture to say that if some young surgeon of an inquiring mind were to choose a place and time when the Honourable and Vigilant Stephen Coleridge was out of the way, and were to produce in a young chimpanzee under an anæsthetic a “greenstick fracture” of his radius and ulna, immobilising it at a right angle for a month, the animal would exhibit at his death some years later a highly developed bursa over the bony protuberance nearly as good as the olecranon bursa on the uninjured side, and better than that of the injured limb. As I have reason to know the meticulous vigilance of this professional and expert humanitarian I hasten here to say in advance that I do not recommend this experiment, not because it would not be entirely justifiable, but because nature herself in the highest Primate has produced many undesigned experiments of nearly equal value, as I hope to show.