Skeletal muscles are structures in which, if ever, the factors of use and habit and disuse would be shown, because muscle is a tissue, with highly active metabolism, so that it has been called “an expensive tissue” for the animal to maintain.

Muscles of Primates.

This physiological fact agrees with the anatomical results of an extended study in the musculature of primates, especially of man, and Hartmann’s book on Anthropoid Apes supplies abundant evidence of the variations of the muscles of these animals, which are not at all more striking than their differing modes of life would suggest. It would be wearisome to quote all these, but a single muscle may be given as an example of a special ape’s muscle with variable distribu­tion. It is called latissimo-condyloideus and starts from the insertion of the latissimus dorsi and passes along the inner aspect of the humerus for a variable distance. In the baboon and others it goes to be inserted into the inner inter-muscular septum and the internal condyle of the humerus, in the orang to the condyle, and in the gibbon to the centre of the shaft. As to origin it proceeds from the insertion of the latissimus dorsi, but in the gorilla from the coracoid process of the scapula and from two portions of the pectoralis minor, and is finally attached to the inter-muscular septum between the brachialis anticus and the triceps; in the chimpanzee it divides into an anterior and posterior portion, the former being attached to the inner condyle, the latter to the middle and inner head of the triceps; in the orang it divides similarly, but in one particular example it had an anterior thin portion attached by a slender tendon to the coracoid process of the scapula and a posterior portion arose from the latissimus dorsi; in the white-handed gibbon it arose from the function of tendons from the latissimus dorsi and teres major and was inserted into the fascia between the tendon of the biceps and the brachialis anticus.

Such a divergence as this within the strict limits of an anthropoid muscle, concerned in the various forms of climbing action of these apes, can only suggest an origin from a divergent set of functions and small details in their respective modes of climbing.

Hand and Foot of Man.

Both the hand and foot of man supply a small muscle for considera­tion in the present connec­tion of habit with formation of new structure. If man be regarded as of simian origin there are not as many entirely new muscles in his equipment as would be expected from his departure from the habits of simian ancestors, though many muscles are found to be altered in size and shifted from the ancestral positions. But the human hand presents one suggestive example of a little muscle not found in any other animal, the special small extensor of the thumb, arising from the interosseous membrane between the radius and ulna, and from the radius, being segmented off from the extensor of the metacarpal of the thumb, and it accompanies this muscle and tendon to be inserted into the first phalanx of the thumb, and is peculiar to man. It can be easily seen at the radial border of the well-known “snuff-box” which is produced by it when it is fully extended. This is of course a muscle of small importance to the functions of the hand, and its appearance in man can only be supposed to be a subordinate detail easily derived from the greater extensor by reason of the more delicate adjustment to complicated movements of the hand under the directing power of higher cerebral development.

Peroneus Tertius.

The foot of man possesses the small peroneus tertius which was referred to as one of the evertors of the foot concerned in the construc­tion of his plantar arch. Macalister and Professor Keith both speak of it as peculiar to man, and the latter refers to it at some length,[79] the whole passage being worth quoting here. “Although the evolution of the human method of progression was attended by a profound altera­tion in the form and action of every muscle and bone with lower limbs, yet this great transforma­tion was produced without the appearance of any really new element. One new muscle—the peroneus tertius—did appear, and the history of its evolution throws an interesting sidelight on the origin of new structures. It arises by the outer fibres of the common extensor muscles of the toes being separated. In all the anthropoids the feet are so articulated at the ankle-joints that the soles are directed towards each other, and only the outer edge of the foot comes to rest on the ground when the animal tries to stand. The feet have a tendency to assume a similar position in children at birth. The advantage of a muscle, such as the peroneus tertius, is apparent in the human foot, for it tends to raise the outer border of the foot, so that the sole is properly applied to the ground. If we examine the muscles which, rising from the front of the leg, cross the ankle-joint to end on the back of the foot on the toes of fifty men, we shall find every stage in the evolution of this muscle. In one man at least it will be undeveloped; in two or perhaps three it will be represented by a part of the tendon of the extensor muscle of the little toe, which in place of ending entirely on the toe sends a part to end on the metatarsal bone of the little toe. In only forty of the fifty men will the peroneus tertius be found quite isolated from the parent muscle—the extensor communis digitorum, and to have a distinct origin from the fibula in the leg, and a separate insertion to the base of the fifth metatarsal bone in the foot. In a series of fifty specimens every stage in the isolation of this new muscle will be seen. It has never been found in any anthropoid, and is more often absent or undeveloped in African than European races.”

To this excellent account I have only to add one comment. It can hardly be an accident or without significance that this special human evertor of the foot concerned in the construc­tion of the plantar arch is “often absent or undeveloped” in African races, which are well-known in some groups to have adapted themselves to a form of foot which shows no plantar arch, being normally flat-footed. In this small field of observa­tion, a mere plot of lentils like that which Shammah defended of old, there is set forth a mimic battlefield, and it is not difficult to see that the forces at work can owe allegiance to one and one only of various commanders. The problem as to the origin of the peroneus tertius would no more attract the Mendelian than did the trousseau and approaching marriage of Caddie Jellyby attract the far-away gaze of her mother, fixed upon the world of Borria-boula-gha, and, for that matter, de Vries would hardly pay it more attention—to him it would be indifferent; whereas Weismann would have as much to say about it as about the little toe of man, which furnished for him and Herbert Spencer such fruitful material for debate many years ago. This muscle resembles the results of some of Michael Angelo’s first attempts at sculpture, thrown aside perhaps in his place of work and from time to time taken up, rough-hewn again and again and finally shaped into a form far from perfect, but with the value and teaching of a failure for him who was some day to outshine all modern rivals. If the history of this muscle be not one of initiative in evolution through the factor of use and habit the Pan-Selectionist must do the best he can with an incalculable number of “trials and errors,” and must suppose that, rather than allow this small territory to the neo-Lamarckian, a long series of man’s ancestors have been making experiments for the benefit of man’s walking power under the guidance of selection with an insignificant muscle whose only function is that of aiding in the eversion of the foot, and that in the rudimentary condition described by Professor Keith it had selective value. No one who was not committed to a dominating theory could hesitate for a moment which of the two alternative views of the origin of the peroneus tertius he would choose. Dr. Barclay Smith speaks in the paper referred to above of the extensor brevis pollicis, or minor, as a muscle of extremely late appearance, and as “peculiarly human,” and says all the evidence points to its being a segmenta­tion product of the extensor ossis metacarpi pollicis, its appearance being foreshadowed in the anthropoid by an extension of that muscle on to the proximal phalanx of the thumb.

It is not without interest to the thesis before us to read the rather bewildering story of the early life of a very insignificant muscle such as the small extensor of the thumb of man.