300 EMBRYOLOGY AND CLASSIFICATION. 



permanent external gills, rank lower than the 

 Salamanders, which lose their gills in the adult 

 condition, while these again are inferior to the 

 Frogs and Toads, in which the tail also is resorbed 

 before the animal completes its growth. But 

 the comparison of the higher and lower Ba- 

 trachians should not stop here. A more exten- 

 sive examination shows that the Tadpole begins 

 as an elongated body, not only without legs, 

 but also without external gills, and that it passes 

 to a branchiate condition, with more or less de- 

 veloped legs, before it loses the gills, while there 

 are various modes of development of the limbs 

 themselves, various phases in the formation 

 of the tail, in its growth and resorption ; vari- 

 ous phases also in the formation of the fingers, 

 up to their final separation, in those which are 

 destitute, in their adult condition, of any web 

 between them. This gradation is so complete, 

 that if we follow all the phases of development 

 of the several representatives of this class, so 

 common everywhere in our temperate zone, we 

 cannot fail to perceive that the changes these 

 animals undergo during their growth furnish a 

 complete scale ; and if we now compare this 

 scale with one founded upon the various degrees 

 of structural complication in the adult repre- 

 sentatives of the class, we find that these two 

 series agree perfectly ; so that Nature herself 



