In this lecture, given in memory of Hugh Owen Thomas, the following points are dealt with:—

(1) It is accepted as proved that the human foot has been evolved from one which was prehensile; the condition found in the foot of the chimpanzee is regarded as the nearest representative of the primitive form from which the feet of man, gorilla and orang have been evolved.

(2) That it is function rather than form which has to be studied if we are to trace aright the sequence of changes which has culminated in the human foot.

(3) That the arch of the human foot is safeguarded and maintained by the reflex postural action of muscles, ligaments being merely second-line defenses. That flat-foot results from a defect in this defense.

(4) The evolution of the various muscles concerned in maintaining the posture of the human foot—the tibialis anticus and tibialis posticus, peroneus longus and peroneus brevis and peroneus tertius—is traced from their state in the prehensile foot of apes to their action in the static foot of man.

(5) It is shown how the grasping muscles—the flexor longus digitorum, the flexor longus hallucis, flexor brevis digitorum and accessorius—become subservient to the static needs of the human foot.

(6) The changes which led to the loss of prehensile action of the great toe and its incorporation in the plantar arch are discussed. It is shown that the change was brought about, not by an adduction of the great toe to the outer toes, but of the outer toes to the great toe.

(7) The changes in structure and action of the short muscles of the great toe are described. These muscles have assumed a new and important function in the human foot,—that of maintaining the static use of the phalangeal part of the great toe.

(8) That the chief changes that have transformed a prehensile into a plantigrade foot are those of growth,—a retrocession of growth affecting the external or plantar limb of the prehensile foot with a progressive growth in its hallucial limb.

(9) That in the evolution of the feet of higher primates four stages have to be recognized: (i) the pronograde prehensile foot; (ii) the hylobatian or small orthograde foot; (iii) the troglodytian or massive orthograde foot, and (iv) the plantigrade or human foot. The foot of man has passed through the first of these three stages in its evolution.

(10) That mass of body has been the most important factor in bringing about the later changes of the feet of the higher primates.

(11) The changes in the gorilla's feet, and their bearing on the evolution of man's foot, in the light of researches carried out recently by Dr. Dudley Morton and Dr. A. Schultz, are discussed.

(12) It is inferred that it was weight of body which compelled man's anthropoid ancestry to assume terrestrial habits of life, and that man is the descendant not of a pigmy anthropoid but one of massive body.

(13) Attention is drawn to the need of recognizing five functional as well as evolutionary areas in the plantar aspect of man's foot.

(14) That orthopaedic surgeons have much to learn by tracing the evolutionary history of the human foot, provided attention is directed not to anatomical details but to the evolution of its postural functions.