CONVERGENT EVOLUTION 451 



changed the more typical Plover-type mainly in the shortening 

 of the tarso-metatarsal bones. The femur or thigh-bone is yet 

 as long as in the more normal Plover. This fact is one of con- 

 siderable interest, affording another link in the chain we are 

 endeavouring to forge to show that change of habits is slow 

 to alter the deep-seated structures of an animal. In the little 

 Diving-Petrels we had an instance of a bird which superficially 

 was much more like an Auk than a Petrel. Yet, in spite of 

 this superficial resemblance, the skeleton has but little changed. 

 The Auks appear to have sojourned on the ocean wastes far 

 longer, and in consequence have become more perfectly adapted 

 to this environment. 



The Auks and the Divers, like the Gulls and the Petrels, 

 were universally believed to be intimately related forms. Yet 

 we know now that they are in no way related. The Divers 

 represent a very ancient group, now almost extinct. They are 

 possibly descendants, or, at any rate, very nearly allied to the 

 giant Hesperornis of the cretaceous epoch. 



Unlike the Diving-Petrels and the Auks, the latter and the 

 Divers {Colymbi) have acquired a more than superficial like- 

 ness ; for the skeletons of the two types present parallels which, 

 while they leave little room for doubt as to their interpretation, 

 at the same time appear to show conclusively that capture of 

 prey under water dates back to an indefinitely long period ; and 

 this is especially true of the Divers, which, among birds that fly, 

 have reached the high-water mark of specialisation to this end. 



As in the Auks, the body has become markedly elongated 

 and depressed, and thereby more perfectly adapted for swim- 

 ming. The legs, in the living bird, seem to leave the body at its 

 extreme hinder end. This fact is due, not to a shifting back- 

 wards of the acetabulum — the socket of the hip girdle for the 

 articulation of the thigh-bone — but to structural peculiarities of 

 the leg itself, all of which contribute towards securing efficiency 

 in swimming, but at the same time render the limb practically 

 useless as an organ of locomotion on land. 



Let us briefly survey these '' expression points " of evolution. 

 The thigh-bone (femur) has been transformed into a much 

 depressed and extremely short bone, and can be rotated for- 

 wards as in other birds. The freedom of movement, however, at 

 the knee-joint has been curtailed in a manner unique among 



