450 A HISTORY OF BIRDS 



term we include the Guillemots, Razor-bills and Puffins — are, as 

 we have already remarked, neither more nor less than highly 

 specialised Plovers. Derived probably from a stock which ob- 

 tained a living entirely from rock-pools and beaches, and later 

 from the sea itself by swimming and diving, the body became 

 more and more transformed to facilitate the pursuit of very 

 active prey under water. We say very active prey advisedly, 

 because certain of the Duck tribe are equally skilled as divers, 

 but they have undergone far less striking modifications, and this 

 apparently because they seek, not agile fish, but algae and 

 mollusca. At the same time it is significant to note that the 

 modifications which these Ducks present are similar in character 

 to those of the Auks. 



The most striking of the changes which the Auks and their 

 kind have undergone are to be found in the thoracic vertebrae, 

 hip-girdle and ribs. 



The first-named have developed relatively enormous spines 

 (hypapophyses) on their under surfaces, and many of these 

 have the free end spread out into j[-shaped processes. These 

 spines afford attachment for highly developed muscles necessary 

 for imparting impetus to the dive. The hip girdle has become 

 ■drawn out, so to speak, posteriorly, so that the ischia, and especi- 

 ally the pubes, form long backwardly directed rods. The 

 hindermost ribs have become enormously elongated so as to 

 touch the free ends of the pubes, and thus form a sort of cage 

 for the viscera. How great is this extension of the ribs can 

 best be appreciated by a comparison of the figure of the trunk 

 skeleton of the aberrant Plover Chionis. This species, a native 

 'Of Arctic regions, has been selected as it is a very ancient form, 

 strictly marine in habitat, and probably nearer the ancestral 

 Plover than the more familiar birds of this genus. 



These changes in the trunk are evidently the outcome of 

 adaptation to an aquatic life. To secure the greatest possible 

 speed in the pursuit of an active prey under water the trunk 

 has become elongated, thereby placing the legs nearer the hinder 

 end of the body, and imparting greater speed and power to the 

 stroke. 



While in the matter of the elongation of the ribs, as will be 

 shown presently, the Auks have exceeded the yet more special- 

 ised Divers, in so far as the legs are concerned they have 



