It  may  be  said  that  this  is  merely  another  way  of  stating
the  principle  of  adaptation  as  the  result  of  natural  selection,
the  most  advantageous  variations  being  selected  at  each
stage  of  life  separately.  But  my  contention  is  that  we  have
no  evidence  that  the  necessary  variations  ever  occurred  until
the  particular  habits  and  conditions  produced  them.  If  it
is  difficult  to  believe  that  the  modifications  were  independent
of  external  conditions,  when  we  compare  different  adapta-
tions in  different  individuals,  it  is  still  more  difficult  when
we  compare  successive  stages  of  the  same  individual,  and
follow  in  actual  progress  the  changes  of  structure  and  function
corresponding  with  change  of  habits  and  conditions.  To  my
mind  the  phenomena  of  metamorphosis  can  only  be  explained
as  due  to  the  heredity  of  modifications  of  growth  originally
caused  directly  in  the  individual  by  changed  conditions
which  introduced  new  irritations,  new  stimulations,  and
changes  of  function.
This  matter  again  can  only  be  studied  in  particular  in-
stances. The  most  familiar  case  is  that  of  the  frog  and  other
amphibia.  We  can  have  no  doubt  that  the  air-breathing
amphibia  were  evolved  from  fishes,  though  we  may  not  be  able


20  SEXUAL  DIMORPHISM

to  say  exactly  what  kind  of  fishes.  We  have,  however,  various
transitional  or  intermediate  forms  in  the  lower  amphibia  and
the  dipnoi  or  lung-fishes,  which  breathe  air  to  some  extent.
