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CHAPTER VII. 



RETROGRESSION. 



THE geological record tells us that, as a rule, 

 the evolution of life proceeded from simple 

 forms to more complex from less to more 

 highly specialised organisms ; but to this 

 order of progression there are numerous 

 exceptions. 



Of these exceptions we propose to deal 

 with two only, but they fully exemplify the 

 problems involved in retrogression. 



An antecessor of the snake was the lizard, 

 an animal provided with four legs for loco 

 motion : the snake has no legs and crawls 

 on its belly. It is thus much less highly 

 specialised than its antecessor. Externally 

 the snake has no trace of limbs, but some 

 species have humeral bones still attached to 

 the skeleton, in others the limb-bones have 

 wholly disappeared. 



On the other hand, the number of the 

 snake's vertebrae has increased. 



