154 INTERESTING CHANGES. 
few vertebrae, that pass quite a period in a larval 
state — that is, having an immature shape that 
will be more or less changed before they reach 
the highest form they are capable of assuming. 
The caterpillar and tadpole are examples of this.” 
“ Oh, yes, I know the tadpole seems to be 
hatched a fish. It has only a round hole for a 
mouth, lives wholly in the water, and breathes 
like them by means of gills.” 
“Yes, but gradually all this changes. The 
mouth widens and becomes adapted to the seizure 
of prey ; the gills disappear, and lungs are de- 
veloped ; and, instead of breathing water, he 
instinctively seeks another element of respiration ; 
legs grow ; the tail, grown useless, disappears ; 
and the metamorphosis is complete. 
“ Now, suppose, at an unfinished period, this 
change should be suspended, and the little being 
should feed and swim on, produce young, and, 
in fact, complete the usual circle of an entire life 
without further development ; and you have an 
idea of what is commonly the history of the little 
batrachian, called the siredon. It is usually a 
permanent larva. Formerly it was supposed, be- 
cause it reproduced in this state, that it could not 
be possible that it ever reached a higher one. 
But Professor Baird, Professor Marsh and others 
have proved that, under favorable circumstances, 
the change is completed, its gills disappear, and 
