STRUCTURAL ADAPTATIONS 385 



small part in the life of the bird little or no adaptation is 

 discernible, as in the Gulls, for example, wherein the hind- 

 limb and pelvic girdle differ in no respect from those of the 

 Plovers. In the fresh-water Ducks, which are more aquatic, 

 the pelvis bears evident signs of modification ; in the Diving- 

 ducks adaptation is still more marked, and has followed along 

 the lines we have already indicated — the backward position of 

 the legs and narrow pelvis, though these changes are by no means 

 so marked as in the Grebes and Divers. 



The maximum of specialisation in this direction was attained 

 by the extinct giant Diver {Hesperomis) (111. 11, p. 36). In this 

 bird the pelvic girdle had assumed all the characters presented 

 by the Divers (Colymbidce) to-day. Of the hind-limb the same 

 may be said, save only in one particular, and that of extreme 

 interest. The difference in question lay in the knee joint, 

 which resembled that of the modern Grebes rather than of the 

 Divers. It resembled the Grebes in that the patella was of 

 enormous size, and differed therefore in that the cnemial crests 

 were not nearly so well developed, so that the disparity in size 

 between' these was far more marked than is the case with the 

 Grebes. Owing to the great size of this patella it was impos- 

 sible for this bird to straighten the leg at the knee joint — the 

 tibia could never have extended further than to form a right 

 angle with the femur, and in this Hesperomis and the marine 

 Divers agree. In Hesperomis adaptation could no further 

 go ; it represents the high-water mark of specialisation in the 

 direction of swimming. Other features following on this 

 specialisation will be dealt with elsewhere. 



Inasmuch as in all the birds so far considered the pursuit of 

 prey under water has resulted in more or less profound modifi- 

 cation of the hind-limb and pelvic girdle, and a decline in the 

 size of the wing, ending in flightlessness in the Grebe of Lake 

 Titicaca, and in the Great Auk, and in the virtual suppression 

 of the wing in Hesperomis, it is a matter for surprise to find 

 that in one group — the Penguins — the incidence of selection 

 has fallen upon the wing, ending in its transformation into a 

 paddle, to all intents and purposes indistinguishable from that 

 of the Cetacea and Turtles among living, and of the Ichthyosaurs 

 and Pleiosaur among extinct, animals. 



In the paddle of the adult Penguin no pollex and no flight- 



35 



