XII REPTILIA, AVES 427 



action of light developing visual organs secondarily, regard the visual 

 character of the pineal organ as being a secondary acquirement evolved 

 independently in the various vertebrates that possess it. 



Birds 



The Birds (AvEs) are in many respects the most highly evolved of 

 existing vertebrates. They seem clearly, as indicated both by the 

 structure of modern birds and by the structure of the oldest known 

 bird Archaeopteryx (Lithographic Stone of the Jurassic Period), to be 

 descendants of reptilian ancestors which developed the power of flight. 



The general form of body is characteristic : the large round head 

 prolonged into a pointed face region, the relatively long flexible neck, 

 the long fore-limbs with the degenerate hand serving merely as an 

 attachment for feathers, the much modified hind-limbs with a greatly 

 elongated ankle region, and the tail reduced to a small stump. 



The act of flying, involving as it does great muscular exertion, 

 necessarily implies intensely active metabolism and this in turn leads to 

 such a production of heat that the temperature of the body is raised to 

 a height considerably above that of its normal surroundings. Birds are 

 therefore warm-blooded, or homoiothermic, to use a more precise term 

 — the important point being not merely that the temperature is higher 

 than that of the surrounding medium but that it remains approximately 

 constant at one level, instead of rising and falling with the external 

 temperature as is the case with the lower — poikilothermic — vertebrates. 



With this physiological peculiarity is correlated a characteristic change 

 in the covering of the body. The scales of the ancestral Reptile have 

 become as it were frayed out to form fluffy feathers which, with the air 

 entangled amongst them, form an admirable non-conducting coat and 

 greatly impede the loss of heat from the body-surface. 



A typical feather (Fig. 184, A) has a highly complicated structure. 

 The central axis is distinguishable into (i) the hollow quill or calamus {c), 

 embedded in a socket or follicle of the skin, perforated at its tip by a 

 minute opening (inferior umbilicus) into which projects a papilla of the 

 dermis provided with blood-vessels and nerves, and containing in its 

 interior a series of little horny caps fitting one over the other, and (2) the 

 rachis (r), white in colour and quadrangular in section. The rachis 

 supports the vane or vexillum — a flattened expansion formed of numerous 

 barbs (b), parallel and adherent to one another, and attached to the 

 rachis at a high angle. Each barb carries two rows of barbules which 

 spring from it very much as the barbs do from the rachis. The distal 



