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  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  194 
  3 
  

  

  at 
  times 
  overstated 
  their 
  exposition 
  of 
  the 
  subject 
  by 
  applying 
  their 
  

   ideas 
  too 
  widely 
  and 
  by 
  insisting 
  too 
  much 
  on 
  one 
  general 
  explanation 
  

   to 
  cover 
  all 
  cases. 
  The 
  fact 
  that 
  the 
  camouflaging 
  of 
  animals 
  is 
  not 
  

   successful 
  at 
  times 
  is 
  not 
  entirely 
  a 
  negation 
  of 
  the 
  camouflaging 
  effect 
  

   of 
  their 
  coloration 
  but 
  may 
  be 
  due 
  to 
  the 
  fact 
  that 
  other 
  senses, 
  such 
  

   as 
  smell, 
  are 
  not 
  affected 
  by 
  the 
  visual 
  results 
  of 
  concealing 
  coloration, 
  

   and 
  offset 
  or 
  render 
  futile 
  the 
  best 
  that 
  camouflage 
  can 
  do. 
  

  

  OBLITERATIVE 
  SHADING 
  

  

  The 
  immediate 
  surroundings 
  in 
  which 
  animals 
  are 
  found 
  are 
  natur- 
  

   ally 
  very 
  variable 
  in 
  such 
  matters 
  as 
  vegetation, 
  amount 
  of 
  light, 
  type 
  

   of 
  earth 
  (whether 
  rocks, 
  gravel, 
  sand, 
  or 
  bare 
  earth 
  are 
  visible, 
  or 
  if 
  

   everything 
  is 
  covered 
  by 
  leafage) 
  , 
  and 
  consequently 
  the 
  patterns 
  needed 
  

   for 
  effective 
  concealment 
  are 
  equally 
  diverse. 
  There 
  is, 
  however, 
  one 
  

   underlying 
  factor 
  common 
  to 
  animals 
  in 
  all 
  of 
  these 
  backgrounds 
  

   to 
  a 
  greater 
  or 
  lesser 
  extent. 
  It 
  is 
  this: 
  regardless 
  of 
  its 
  particular 
  

   color 
  pattern, 
  to 
  become 
  relatively 
  invisible 
  an 
  animal 
  must 
  lose 
  its 
  

   appearance 
  of 
  solidity, 
  or, 
  to 
  put 
  it 
  in 
  other 
  words, 
  must 
  not 
  appar- 
  

   ently 
  cast 
  a 
  shadow 
  on 
  itself. 
  The 
  light 
  falling 
  on 
  an 
  animal 
  usually 
  

   comes 
  from 
  one 
  direction, 
  generally 
  from 
  above, 
  so 
  that 
  some 
  parts 
  

   of 
  the 
  animal 
  (usually 
  its 
  back) 
  are 
  in 
  stronger 
  light 
  and 
  the 
  opposite 
  

   parts 
  (usually 
  the 
  underside) 
  are 
  in 
  dimmer 
  light 
  or 
  even 
  in 
  the 
  

   shadow 
  of 
  the 
  illuminated 
  parts 
  and 
  tend 
  to 
  look 
  darker. 
  This 
  is 
  

   easily 
  seen 
  by 
  placing 
  a 
  white 
  ball 
  on 
  a 
  table 
  with 
  the 
  ligh 
  coming 
  

   from 
  above 
  — 
  the 
  under 
  surface 
  of 
  the 
  ball 
  is 
  shaded 
  and 
  at 
  once 
  re- 
  

   veals 
  the 
  spherical 
  solidity 
  of 
  the 
  ball, 
  even 
  though 
  it 
  be 
  placed 
  on 
  

   a 
  white 
  table. 
  In 
  most 
  animals 
  the 
  light 
  and 
  dark 
  tones 
  are 
  so 
  ar- 
  

   ranged 
  that 
  they 
  somewhat 
  counteract 
  the 
  effect 
  of 
  self-shadowing. 
  

   This 
  is 
  brought 
  about 
  by 
  having 
  the 
  darker 
  tones 
  where 
  the 
  light 
  

   strikes 
  (usually 
  from 
  above) 
  and 
  the 
  paler 
  tones 
  on 
  the 
  parts 
  in 
  

   shadow 
  (usually 
  the 
  lower 
  parts). 
  In 
  other 
  words, 
  darker 
  tones 
  plus 
  

   more 
  light 
  on 
  one 
  side 
  tends 
  to 
  equalize 
  paler 
  tones 
  plus 
  shadow 
  on 
  

   the 
  other. 
  The 
  result 
  is 
  a 
  greater 
  or 
  lesser 
  degree 
  of 
  reduction 
  of 
  the 
  

   visible 
  solidity 
  of 
  the 
  animal. 
  This 
  distribution 
  of 
  light 
  and 
  dark 
  

   tones 
  on 
  the 
  animal, 
  tending 
  to 
  counterbalance 
  the 
  unequal 
  lighting 
  

   the 
  parts 
  receive, 
  is 
  known 
  as 
  obliterative 
  shading 
  or 
  countershading. 
  

  

  Countershading 
  is, 
  therefore, 
  a 
  basic 
  principle 
  of 
  animal 
  coloration 
  

   and 
  is 
  of 
  wide 
  occurrence 
  in 
  nature. 
  Many 
  and 
  quite 
  unrelated 
  

   groups 
  of 
  animals 
  — 
  mammals, 
  birds, 
  reptiles, 
  fishes, 
  etc. 
  — 
  in 
  all 
  parts 
  

   of 
  the 
  world 
  show 
  it. 
  Countershading 
  may 
  be 
  described 
  in 
  relation 
  

   to 
  body 
  form, 
  to 
  environment, 
  and 
  to 
  habits. 
  In 
  some 
  fishes 
  with 
  

   deep-bodied, 
  laterally 
  compressed 
  forms 
  having 
  nearly 
  vertical 
  sides 
  

   with 
  but 
  very 
  slight 
  convex 
  curvature, 
  strong 
  countershading 
  would 
  

   defeat 
  its 
  own 
  end, 
  and 
  it 
  is 
  noteworthy 
  that 
  such 
  species 
  are 
  only 
  

  

  