384 A HISTORY OF BIRDS 



The skeleton of the leg has undergone no less striking 

 changes. With an extremely shortened thigh-bone, and per- 

 manently flexed at the knee joint, all that appears of the leg 

 externally is the scale-covered tarso-metatarsus and the toes. 

 This free portion leaves the body at its extreme hinder end, 

 and in consequence the bird when standing carries the body 

 erect instead of horizontal. At least this is true of the fresh- 

 water Grebes, but the marine Di\'ers {Colymbidce) can scarcely 

 stand upon the legs, owing to the fact that the toes ha\'e lost 

 the power of bending forwards upon the shank of the tarso- 

 metatarsus. 



In the skeleton of this limb, as we have just remarked, the 

 femur has become extremely shortened, and flattened dorso- 

 ventrally. Its lower end articulates as usual with the shaft of 

 the tibio-tarsus, but the latter cannot be extended in the same 

 straight line as the femur owing to the fact the cnemial crests, 

 or bony ridges for the attachment of the muscles of the lower 

 leg, have grown upwards to an enormous extent, so that in the 

 Diver a deep groove has to be carved therein to receive the thigh- 

 bone, as may be seen in the accompanying figure. The patella 

 or knee cap is represented only by a vestige in the shape of a 

 small flake of bone. In the Grebe the cnemial crest is rather 

 less developed, but the end of the thigh-bone is gripped much 

 as in the Divers : not, however, by the overgrowth of the 

 cnemial crest, but by an enormous patella which is firmly held 

 by a ligament to the crest just referred to, above which it projects 

 some distance. Inasmuch, however, as these Hgaments are 

 elastic a certain amount of play is possible at the knee joint, 

 sufficient to allow of the leg being straightened out far enough 

 to make standing or even walking possible. 



The tarso-metatarsus in these birds is extremely compressed 

 from side to side presenting a knife-like edge for the cleavage of 

 the water when swimming, and this same lateral compression 

 is to be found in the toes, so that when the foot is closed at the 

 end of the stroke the toes lie one behind the other so as to 

 offer the least possible resistance in bringing the foot forwards 

 for the next stroke. 



The gradual evolution of this extremely specialised type of 

 girdle and limb may be studied in a survey of these organs in 

 water-birds of various types. Where swimming plays only a 



