Fig. 48.—Swan, in the act of swimming, the right foot being fully expanded, and about to give the effective stroke, which is delivered outwards, downwards, and backwards, as represented at r of fig. 50; the left foot being closed, and about to make the return stroke, which is delivered in an inward, upward, and forward direction, as shown at s of fig. 50. In rapid swimming the swan flexes its legs simultaneously and somewhat slowly; it then vigorously extends them.—Original.
Fig. 49.—Foot of Grebe (Podiceps). In this foot each toe is provided with its swimming membrane; the membrane being closed when the foot is flexed, and expanded when the foot is extended. Compare with foot of swan (fig. 48), where the swimming membrane is continued from the one toe to the other.—(After Dallas.)
One of the most exquisitely constructed feet for swimming and diving purposes is that of the grebe (fig. 49). This foot consists of three swimming toes, each of which is provided with a membranous expansion, which closes when the foot is being drawn towards the body during the back stroke, and opens out when it is being forced away from the body during the effective stroke.
Fig. 50.—Diagram representing the double waved track described by the feet of swimming birds. Compare with figs. [18] and [19], pp. 37 and 39, and with fig. [32], p. 68.—Original.
In swimming birds, each foot describes one side of an ellipse when it is extended and thrust from the body, the other side of the ellipse being described when the foot is flexed and drawn towards the body. The curve described by the right foot when pushed from the body is seen at the arrow r of fig. 50; that formed by the left foot when drawn towards the body, at the arrow s of the same figure. The curves formed by the feet during extension and flexion produce, when united in the act of swimming, waved lines, these constituting a chart for the movements of the extremities of swimming birds.
There is consequently an obvious analogy between the swimming of birds and the walking of man (compare fig. 50, p. 97, with fig. [19], p. 39); between the walking of man and the walking of the quadruped (compare figs. [18] and 19, pp. 37 and 39); between the walking of the quadruped and the swimming of the walrus, sea-bear, and seal; between the swimming of the seal, whale, dugong, manatee, and porpoise, and that of the fish (compare fig. [32], p. 68, with figs. [18] and 19, pp. 37 and 39); and between the swimming of the fish and the flying of the insect, bat, and bird (compare all the foregoing figures with figs. [71], [73], and [81], pp. 144 and 157).