OBGANIZATION OF ANIMALS. 195 



It were easy to give many illustrations of this fact: two 

 distinct plans of organization are obvious in the lizard and 

 the carp ; they differ in the general conformation of the body, 

 their kind of life, their mode of respiration and circulation ; 

 but the salamanders, the axolotls (Fig. 145), the lepidosirens 



Fig. 145. The Axolotl, of Mexico. (Siren pisciforme.) 



(Fig. 146), and some other animals, present us with modes 

 of organization intermediate to these two types, and establish 

 transitions so gradual from one to the other as to make it 

 difficult to determine whether the animal in question be a 

 batrachian or a fish. These transitions from one animal to 



Fig. 146. The Lepidosiren. (Potnpterus.) 



another are not limited to the comparing of two distinct 

 adult animals ; they may be observed in comparing the same 

 animal at different stages of its growth or development. 

 Frogs, for example, present at birth nearly all the characte- 

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