452 A HISTORY OF BIRDS 



birds. This has come about by the upgrowth in a semicircle 

 of the articular surface of the tibia so as to grasp the end of 

 the femur in a close embrace, absolutely preventing the 1^ 

 being straightened. Thus, the tibia in the living bird is per- 

 manently flexed on the femur at more or less of a right angle. 

 In swimming when the thigh is brought forwards for the com- 

 mencement of the stroke the leg-shank is brought to a position 

 at right angles to the long axis of the body ; at the end of the 

 stroke it lies parallel with the long axis thereof. But this 

 tibia is still further remarkable for the enormous development 

 of the cnemial crest which rises far above the level of the thigh- 

 bone. The patella, in consequence, has become reduced to a 

 mere scale, partaking in shape of the form of the crest behind 

 which it lies. This huge crest affords attachment to powerful 

 swimming muscles. 



In the hind-limb of the lacustrine Grebes we find a mid- 

 way stage in the development of this remarkable joint. Here 

 a similar tibial semicircular socket is formed, not, however, by 

 the semicircular growth of the tibial articular surface, but by a 

 large patella which allows of a little more movement. As a 

 consequence the Grebes can walk, though with difificulty, on 

 land ; the Divers, as we have already remarked, cannot. On 

 land these birds can sit upright, but locomotion can only be 

 effected by lying along the belly and shoving the body for- 

 ward by means of the feet. As in the Divers, the cnemial 

 processes are enormously developed, though they do not attain, 

 relatively, so large a size, and are overtopped by the great 

 pyramidal patella, which in the Divers has become suppressed 

 by the great cnemial crest. 



The tarso-metatarsus and the toes, which together make 

 the foot, is in both Divers and Grebas extremely compressed, 

 offering little more than a knife-edge to the water when swim- 

 ming. In both Grebes and Divers the outer toes are so arranged 

 as to fold up one behind the other at the end of the stroke 

 when the leg is being brought forwards. The outer toe is the 

 longest in both forms. The Divers are web-footed, but in the 

 Grebes broad lobes along each toe take the place of webs. 

 This is really a very curious fact and demands closer scrutiny. 

 It has already caused comment, some inclining to the belief 

 that the Grebes are- — and on this account — really more nearly 



