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 
