STRUCTURE AND DEVELOPMENT 45 



useless vestiges, which, in the latter case, are hidden 

 under the brushhke feathers covering the body. It 

 is unnecessary to add more examples, for even these 

 few illustrations estabhsh exactly the same i)rincii)les 

 of relationship and evidences of evolution that arc to 

 be found in the series of mammalia. 



Reptiles also are grouped, like the mammals and birds, 

 as variations about a central theme. An ordinary lizard 

 is perhaps the nearest in form to the remote ancestor 

 from which all have sprung. Some lizards are lung 

 and very slender, with all four limbs of greatly reduced 

 size. Others, which are still true lizards, have lost the 

 hind limbs, or even all the legs, as in the ''blind worms" 

 of England. One step more, and an animal which has 

 progressed further along a similar line of descent 

 would be a snake. Just as whales as a group are 

 derivable from forms which resemble types belonging 

 to another order, so snakes as an order are to be regarded 

 as more radically altered derivatives of some four-footed 

 lizardlike creature. Alligators are very much like 

 lizards in general form, and their order is a diverging 

 branch from the same limb. Finally the evolution of 

 turtles from the same ancestors is intelligible if we begin 

 with a short stout animal like the so-called "horned 

 toad" of Arizona, and proceed to the soft-shelled 

 tortoise of the Mississippi River system ; the establish- 

 ment of a bony armor completes the evolution of the 

 familiar and more characteristic turtle. 



Frogs and salamanders constitute another lower 

 class, called the amphibia, whose members are gilled 

 during the earlier stages of development. An adult 

 frog is essentially a salamander without a tail and 



