322 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



entirely oh its legs, not only for support, but also for 

 means of locomotion, the number of vertebrae that so 

 fuse together may be greater than twenty. Chevron 

 bones are sometimes, as in lizards and crocodiles, de- 

 veloped in the caudal region, the length of which 

 varies considerably among reptiles ; but in flying 

 birds, where a tail would seriously affect the centre of 

 gravity, the caudal vertebrae are reduced in number, 

 and unite to form the ploughshare bone, with which 

 the so-called rectrices, or steering feathers, are con- 

 nected (Fig. 135; co). An exception to the rule that 

 the central portion of the vertebrae is that which is 

 most completely ossified is afforded by the caudal ver- 

 tebrae of many lizards. In these there is a central 

 unossified region, where, of course, the tail is much 

 weaker than at other points. If a lizard be seized by 

 the tail it will ordinarily escape, thanks to the fracture 

 of one of these centra, while the part of the tail left 

 with the lizard will grow again. Here we have, no 

 doubt, an example of a variation which has been 

 seized upon, on the principle of natural selection, and 

 has afforded these long-tailed but inoffensive forms 

 with a satisfactory, though undignified, method of 

 protection and escape. 



In fishes the vertebral column can only be 

 divided into that which belongs to the trunk and that 

 which belongs to the tail. In some of the more gene- 

 ralised the notochord extends in a straight line to the 

 hinder end of the body, dividing the tail-fin into two 

 equal halves. In most Elasmobranchs this notochord 

 is bent upwards, and the lower half of the fin is much 

 larger than the upper. In the Teleostei the 

 notochord likewise becomes so bent up, but the rays 

 which support the fin become so arranged as to give 

 to the tail fin, when seen from the surface, the appear- 

 ance of being composed of equal upper and lower 

 halves, though, as a matter of fact, all the fin rays are 



