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  BRITISH 
  LEPIDOPTERA. 
  

  

  conditions 
  of 
  food, 
  mode 
  of 
  life, 
  habit, 
  etc., 
  in 
  other 
  words, 
  that 
  these 
  

   local 
  races 
  themselves 
  vary 
  in 
  the 
  extent 
  to 
  which 
  they 
  have 
  travelled 
  

   towards 
  species, 
  for 
  we 
  have 
  no 
  doubt 
  that 
  the 
  internal 
  physiological 
  

   variations 
  are 
  often 
  of 
  much 
  greater 
  importance 
  in 
  the 
  making 
  and 
  

   fixation 
  of 
  species 
  than 
  the 
  external 
  differences 
  which 
  are 
  so 
  patent. 
  

   These 
  external 
  differences, 
  on 
  which 
  the 
  systematist 
  separates 
  his 
  

   species, 
  are 
  often 
  merely 
  outward 
  marks 
  of 
  a 
  much 
  more 
  fundamental 
  

   internal 
  variation 
  of 
  structure 
  and 
  function, 
  difficult 
  to 
  observe 
  and 
  

   still 
  more 
  difficult 
  to 
  demonstrate. 
  Whilst, 
  therefore, 
  the 
  general 
  

   principle 
  may 
  be 
  admitted, 
  that 
  crossbreeding 
  between 
  males 
  and 
  

   females 
  of 
  the 
  same 
  species 
  that 
  have 
  been 
  subjected 
  to 
  different 
  

   environmental 
  conditions 
  resulting 
  only 
  in 
  superficial 
  differences, 
  

   usually 
  results 
  in 
  greater 
  vigour 
  and 
  fertility, 
  it 
  must 
  not 
  be 
  overlooked 
  

   that, 
  if 
  functional 
  and 
  internal 
  structural 
  changes 
  have 
  also 
  taken 
  

   place, 
  the 
  results 
  may 
  be 
  entirely 
  contrary 
  to 
  those 
  expected, 
  and 
  that 
  

   a 
  lower 
  grade 
  of 
  vigour 
  and 
  fertility, 
  if 
  not 
  a 
  real 
  approach 
  to 
  sterility, 
  

   so 
  frequently 
  found 
  in 
  the 
  crossing 
  of 
  true 
  species, 
  may 
  be 
  presented. 
  

   In 
  this 
  way, 
  the 
  crossing 
  of 
  races 
  of 
  the 
  same 
  species 
  may 
  give 
  very 
  

   different 
  results 
  in 
  different 
  species, 
  and 
  their 
  study 
  may 
  help 
  much 
  to 
  

   solve 
  some 
  of 
  the 
  difficult 
  problems 
  growing 
  out 
  of 
  the 
  subject 
  of 
  

   hybridism. 
  

  

  We 
  have 
  suggested 
  here 
  that 
  internal 
  differences 
  of 
  structure 
  and 
  

   function, 
  changes 
  in 
  mode 
  of 
  life, 
  food, 
  and 
  habit, 
  etc., 
  have 
  brought 
  

   about 
  the 
  separation 
  of 
  specific 
  forms, 
  and 
  that 
  closely-allied 
  species 
  

   are 
  more 
  often 
  due 
  to 
  a 
  differentiation 
  of 
  function 
  or 
  habit 
  than 
  to 
  any 
  

   marked 
  external 
  structural 
  character, 
  and 
  hence 
  we 
  find 
  that, 
  when 
  

   local 
  races 
  begin 
  to 
  offer 
  marked 
  physiological, 
  i.e., 
  functional, 
  differ- 
  

   ences, 
  the 
  races 
  often 
  lose 
  that 
  invigorating 
  power 
  that 
  crosses 
  of 
  more 
  

   nearly 
  related 
  local 
  forms 
  present, 
  and 
  produce 
  offspring 
  that 
  are 
  in 
  some 
  

   degree 
  sterile. 
  But, 
  although 
  this 
  may 
  be 
  conceded, 
  we 
  must 
  be 
  careful 
  

   not 
  to 
  assert 
  that, 
  because 
  two 
  assumed 
  species 
  being 
  crossed 
  do 
  produce 
  

   fertile 
  progeny, 
  the 
  two 
  species 
  are 
  not 
  in 
  reality 
  distinct 
  species, 
  but 
  

   only 
  forms 
  of 
  the 
  same 
  species. 
  The 
  attempt 
  to 
  make 
  sterility 
  the 
  

   test 
  of 
  species 
  has 
  already 
  been 
  put 
  forward 
  ; 
  it 
  can 
  only, 
  however, 
  be 
  

   considered 
  as 
  one 
  of 
  many 
  other 
  tests 
  of 
  specific 
  distinction. 
  

  

  In 
  dealing 
  with 
  this 
  subject 
  it 
  is 
  well 
  to 
  keep 
  clearly 
  in 
  mind 
  the 
  

   distinction 
  between 
  crosses 
  of 
  distinct 
  species, 
  resulting 
  in 
  the 
  pro- 
  

   duction 
  of 
  hybrids, 
  and 
  crosses 
  of 
  varieties 
  (local 
  races) 
  of 
  the 
  same 
  

   species, 
  resulting 
  in 
  the 
  production 
  of 
  mongrels. 
  The 
  scientific 
  

   utility 
  of 
  the 
  results 
  hitherto 
  obtained 
  by 
  crossing 
  two 
  forms 
  of 
  a 
  

   species, 
  one 
  of 
  which 
  is 
  more 
  or 
  less 
  typical, 
  and 
  the 
  other 
  merely 
  a 
  

   sporadic 
  aberration, 
  or 
  a 
  form 
  which 
  has 
  not 
  yet 
  developed 
  into 
  a 
  

   distinct 
  local 
  race, 
  is 
  very 
  problematical, 
  and 
  the 
  results 
  are, 
  probably, 
  

   so 
  far 
  as 
  their 
  bearing 
  on 
  the 
  broad 
  questions 
  underlying 
  hybridity 
  

   is 
  concerned, 
  of 
  little 
  more 
  importance 
  than 
  of 
  breeding 
  any 
  two 
  other 
  

   specimens 
  of 
  the 
  same 
  species 
  together. 
  That 
  the 
  sporadic 
  aberration 
  

   happens 
  to 
  have 
  a 
  marked 
  peculiarity 
  of 
  colour 
  or 
  form 
  (due 
  possibly 
  

   to 
  a 
  matter 
  of 
  larval 
  health) 
  may 
  be 
  nothing, 
  and, 
  so 
  far 
  as 
  this 
  question 
  

   is 
  concerned, 
  such 
  an 
  aberration 
  is 
  not 
  to 
  be 
  considered 
  on 
  the 
  same 
  

   level 
  as 
  a 
  local 
  race. 
  The 
  crossing 
  of 
  different 
  forms 
  of 
  a 
  dimorphic, 
  

   trimorphic, 
  or 
  polymorphic 
  species, 
  the 
  two, 
  three, 
  or 
  more 
  forms 
  being 
  

   developed 
  naturally, 
  in 
  the 
  same 
  locality, 
  and 
  occurring 
  in 
  the 
  same 
  

   brood, 
  must 
  be 
  placed 
  in 
  a 
  different 
  category, 
  but 
  the 
  most 
  that 
  can 
  be 
  said 
  

  

  