He is hardly prepared to say that its greater vivacity may not be the expression of more rapid oxidation; but he sees no fore-ordained balance of vital enterprise and chemical change. He is, or ought to be, extremely suspicious of any explanation which appears to over-ride physical laws; yet, at the same time, he is aware that until he has more accurate knowledge regarding the constitution of protoplasm he will not be in a position to understand how physical laws apply. The protoplasmicity of protoplasm is increased by warmth. What change of molecular constitution does this imply? The view that in a muscle molecular change gives rise to an electrical change, which in turn produces the change in form, has been very widely held. The hypothesis was based on observations which seemed to show that the electric variation travels a little ahead of the wave of contraction; but every improvement in recording apparatus has diminished this apparent want of synchronism. There can be little doubt but that the lagging behind of the wave of contraction is due to the inertia of the muscle and of the recording apparatus. Molecular change and electric variation are simultaneous. If this be true, the electric change cannot be regarded as the cause of the molecular change, in the sense, at any rate, in which they used to be considered as cause and effect. The =power of muscle= varies as its cross-section. For human muscles the maximum lift amounts to from 7 to 10 kilogrammes for each square centimetre. This is a large figure, but it must be remembered that, owing to the arrangement of the bones as levers, most muscles act at a great mechanical disadvantage. The greater the difference in distance from the fulcrum between the point of application of the force and the point of incidence of the weight, when the force acts nearer to the fulcrum than the weight, the greater is the mechanical disadvantage. The greater also is the rapidity with which the weight is lifted. What is lost in strength is gained in swiftness. Contrast the slow steps of a negro, whose long heel separates the point of application of the power (tendo Achillis) from the fulcrum (the ankle-joint), with the springy movements of a European. A European needs, and as a rule has, a better developed calf, which allows him his more sprightly gait, without sacrificing his carrying power. Our preference for slender wrists and ankles is not purely æsthetic, unless we admit, as may be maintained, that all natural canons of taste rest upon utility. Slimness of joints means nimbleness. A few muscles act directly, without loss of power—as, for example, the masseter, which lifts the lower jaw (hence a grand capacity for cracking nuts)—but most muscles move levers of considerable length. Compare with the masseter the biceps and brachialis which lift the forearm. Their tendons are inserted into the radius and the ulna at a distance from the elbow-joint which is about one-tenth as great as the distance from