PROGRESSION ON AND IN THE WATER. 91 



wing into a powerful oar for swimming, it is only necessary 

 to extend and flex it in a slightly backward direction, the 

 mere act of extension causing the feathers to roll down, and 

 giving to the back of the wing, which in this case communi- 

 cates the more effective stroke, the angle or obliquity neces- 

 sary for sending the animal forward. This angle, I may 

 observe, corresponds with that made by the foot during ex- 

 tension, so that, if the feet and wings are both employed, 

 they act in harmony. If proof were wanting that it is the 

 back or convex surface of the wing which gives the more 

 effective stroke in subaquatic flight, it would be found in the 

 fact that in the penguin and great auk, which are totally in- 

 capable of flying out of the water, the wing is actually twisted 



Fig. 46. — The Little Penguin (Aptenodytes minor y Linn.), adapted exclusively 

 for swimming and diving. Tn this quaint Dird the wing forms a perfect 

 screw, and is employed as such in swimming and diving. Compare with 

 fig. 37, p. 70, and tig. 44, p. 89.— Original. 



round in order that the concave surface, which takes. a better 

 hold of the water, may be directed backwards /fig.. 4 6).^ The' 

 thick margin of the wing when giving the effective stroke 

 is turned downwards, as happens in the flippers of the 

 sea-bear, walrus, and turtle. This, I need scarcely remark, is 

 precisely the reverse of what occurs in the ordinary wing in 

 aerial flight. In those extraordinary birds (great auk and 

 penguin) the wing is covered with short, bristly-looking 

 feathers, and is a mere rudiment and exceedingly rigid, the 



^ In the swiniraing of the crocodile, turtle, triton, and frog, the concave 

 surfaces of the feet of the anterior extremities are likewise turned backwards. 



