180 THE DARWINIAN THEORY 



existing reptiles the lizards are the most primitive. 

 Snakes are comparatively recent tertiary descendants 

 of lizards, characterised by the absence or vestigial 

 condition of the limbs. Crocodiles and tortoises both 

 date back to a very remote period. 



Aves. — Birds are a very special group, character- 

 ised by the possession of feathers, and the conversion 

 of the fore-limb into a wing. Their bones are very 

 light, with a marked tendency to fusion, as seen in 

 the metacarpal and metatarsal bones. The greater 

 number of their distinguishing points are obviously 

 correlated with the power of flight. Birds are warm- 

 blooded and really terrestrial, and form a compact, 

 well-developed group, at first sight very independent, 

 and having no obvious kinship with any one group 

 of Vertebrates rather than another. The affinities of 

 birds we have already discussed, and Palaeontology 

 speaks very decidedly on the point. There are no 

 forms directly intermediate between birds and 

 reptiles, but such forms, we have seen, should not 

 exist. 



We find reptiles which gradually lose the distinctive 

 reptilian characters, and birds which, as we proceed 

 backwards in time, gradually show less and less the 

 special features separating birds from reptiles. 



This is shown in the case of the pelvic girdle ; the 

 ■pubes in the crocodile pointing downwards and 

 forwards, in the bird downwards and backwards ; 

 ' while in the Dinosaurus both processes are present, 

 and one is preserved in birds, the other in reptiles. 

 The Dinosaurus therefore forms a true link between 

 birds and reptiles; and this is supported by the 



