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. [Illustration: 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). [Illustration: FIG. 51.--The Flying-fish (_Exocœtus exsiliens_, Linn.), with wings expanded and elevated in the act of flight (_vide_ arrows). This anomalous and interesting creature is adapted both for swimming and flying. The swimming-tail is consequently retained, and the pectoral fins, which act as wings, are enormously increased in size.--_Original._]