Amphibia. 195 



breathes by means of lungs. The frog goes through all these stages, 

 but rises to a still higher level by getting rid of the tail which it had in 

 its larval life. In short, each of these forms has gone through all the 

 stages represented by the forms below it in the scale, but has discarded 

 certain features and has advanced to a higher plane and leads a more 

 active life. 



The life history of the frog, therefore, serves to review all the other 

 forms of amphibians below it in the scale. The temporary stages of the 

 frog's life represent the permanent form of the lower kinds of Amphibia. 

 To put it in another way, they all start at the foot of the same stairs 

 at the fish level, so to speak ; the siren climbs up on the first step and 

 stops there ; the mud puppy makes one more step and rests content ; 

 the salamander mounts still higher by a step and has reached its high- 

 est point ; while the frog takes all of these steps and reaches out once 

 again and tops the series by getting on the highest step of the stairs. 



This series illustrates a law which, though general, does not appear 

 so clear in some other groups. The stages of development of the 

 individuals of the higher groups recapitulate the development of the 

 group as a whole; or, in other words, "the development of the individ- 

 ual epitomizes the development of the race. 11 



DIFFERENCES BETWEEN FISHES AND AMPHIBIANS. 



FISHES. AMPHIBIANS. 



Persist through life Gills . . . May disappear in adult 



Air bladder respiratory in lungfishes . Lungs . . . Present in adult 

 i Auricle, i Ventricle .... Heart . . .2 Auricles, i Ventricle 



Fins with fin rays Limbs . . . Limbs with digits 



Scales (usually) .... Exoskeleton . . Skin smooth (usually) 

 Not open to mouth (except lungfish) Nostrils . . . Open into mouth 



ORDERS OF AMPHIBIA. 



C Order i. Urodela .... Salamander 



Class Amphibia . . . J Order 2. Anura Frog 



I Order 3. Gymnophiona . . . Blind-snake 



