PROGRESSION ON AND IN THE WATER. 



97 



feet more or less tilted. The same movements are seen in the 

 feet of the swan, and in those of swimming birds generally 

 (fig. 48).^ 



One of the most exquisitely constructed feet for swimming 

 and diving purposes is that of the grebe (fig. 49). This foot 



Fig. 49.— Foot of Grebe (Podiceps). In this foot each toe is provided with its 

 swimming membrane ; the membrane being closed wlien tiie 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.) 



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 Avave<l track described by the feet 

 of swimming birds. Compare with figs. 18 and 19, pp. 3T and 39, and with 

 32, li. {"iS.— 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 



