THE EVOLUTIONIST AT LARGE. 



of an original reptilian or half-reptilian an- 

 cestor. 



\Vhy modern birds have lost their long 

 flexible tails it is not difficult to see. The tail 

 descends to all higher vertebrates as an heir- 

 loom from the fishes, the amphibia, and their 

 other aquatic predecessors. With these it is 

 a necessary organ of locomotion in swim- 

 ming, and it remains almost equally useful to 

 the lithe and gliding lizard on land. Indeed, 

 the snake is but a lizard who has substituted 

 this wriggling motion for the use of legs 

 altogether ; and we can trace a gradual suc- 

 cession from the four-legged true lizards, 

 through snake-like forms with two legs and 

 wholly rudimentary legs, to the absolutely 

 limbless serpents themselves. But to flying 

 birds, on the contrary, a long bony tail is 

 only an inconvenience. All that they need 

 is a little muscular knob for the support of 

 the tail-feathers, which they employ as a 

 rudder in guiding their flight upward or 

 downward, to right or left The elongated 



