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