The  subaqueous  life  of  fishes  has  led  to  the  modification  of
their  two  pairs  of  limbs  into  fins,  although  the  precise  analogy
of  these  organs  to  the  legs  of  terrestrial  mammals  and  reptiles
may  easily  be  lost  sight  of  in  certain  genera  and  species  by  the
development  of  unpaired,  vertical,  or  median  fins,  which  some-
times eclipse  in  size  and  prominence  the  original  limbs,  as  in
the  perch  (Fig.  I.,  75).  Sometimes  all  external  trace  of  the
original  limbs  is  lost,  as  in  the  eel,  which  has  no  ventral  fins
representing  the  hind  legs,  but  has  developed  instead  a  con-
tinuous, confluent  median  fin,  extending  longitudinally  over
the  greater  part  of  both  upper  and  lower  surfaces.  The  total
disappearance  in  the  eel  of  those  limbs  which  perform  the
office  of  propulsion  in  terrestrial  animals  is  the  more  remark-
able when  it  is  considered  that,  of  all  British  fishes,  this  is  the
only  one  that  ever  voluntarily  leaves  the  water  to  travel  by
land.  The  eel,  however,  retains  fully-developed  pectoral  fins,
the  homologues  of  arms  in  man.
Ray  and  Willughby,  two  excellent  naturalists  and  collabo-
rators of  the  seventeenth  century,  arrived  at  a  definition  of
fishes  which  we  recognise  as  perfectly  sound  and  complete  at
this  day.  In  the  class  of  Fishes  they  placed  all  animals  with
blood,  breathing  by  gills,  provided  with  a  single  ventricle  of
the  heart,  and  wholly  or  partly  covered  with  scales,  or  naked.
But  the  exclusion  of  whales  from  the  class  of  Fishes  involved
in  this  definition  proved  so  startling  and  unacceptable  to  the
scientific  men  of  the  day,  that  Ray  afterwards  erroneously
modified  his  terms  so  as  to  include  warm-blooded  animals
inhabiting  the  water.
