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  ANIMAL 
  BEHAVIOUR 
  

  

  " 
  Each 
  stimulus 
  causes 
  as 
  a 
  rule 
  not 
  merely 
  a 
  single 
  definite 
  

   action 
  that 
  may 
  be 
  called 
  a 
  reflex, 
  but 
  a 
  series 
  of 
  ' 
  trial 
  ' 
  

   movements, 
  of 
  the 
  most 
  diverse 
  character, 
  and 
  including 
  at 
  

   times 
  practically 
  all 
  the 
  movements 
  of 
  which 
  the 
  animal 
  is 
  

   capable" 
  (Jennings, 
  1906, 
  p. 
  280). 
  

  

  6. 
  Tropisms. 
  

  

  From 
  chains 
  of 
  reflexes, 
  suffused 
  with 
  awareness, 
  it 
  is 
  not 
  

   difficult 
  to 
  pass 
  to 
  the 
  level 
  of 
  instinctive 
  behaviour, 
  but 
  

   before 
  we 
  pass 
  to 
  that 
  level 
  we 
  have 
  to 
  recognise 
  the 
  important 
  

   role 
  played 
  by 
  tropisms 
  (see 
  Loeb, 
  1918). 
  Tropisms 
  are 
  

   obligatory 
  or 
  forced 
  movements 
  of 
  the 
  creature 
  as 
  a 
  whole, 
  

   which 
  more 
  or 
  less 
  automatically 
  secure 
  physiological 
  equi- 
  

   librium 
  in 
  relation 
  to 
  outside 
  stimuli, 
  such 
  as 
  light 
  or 
  heat, 
  

   gravity 
  or 
  electricity, 
  diffusing 
  chemicals 
  or 
  water-currents. 
  

   When 
  a 
  moth, 
  constitutionally 
  adapted 
  to 
  nocturnal 
  activity, 
  

   comes 
  in 
  its 
  flight 
  within 
  the 
  sphere 
  of 
  influence 
  of 
  a 
  candle, 
  

   and 
  has 
  one 
  eye 
  much 
  more 
  illumined 
  than 
  the 
  other, 
  owing 
  

   to 
  the 
  direction 
  in 
  which 
  it 
  happens 
  to 
  be 
  flying, 
  more 
  intense 
  

   chemical 
  processes 
  are 
  set 
  up 
  in 
  the 
  illumined 
  eye. 
  On 
  that 
  

   side 
  there 
  is 
  therefore 
  a 
  relative 
  increase 
  in 
  the 
  mass 
  of 
  

   certain 
  chemical 
  products. 
  But 
  messages, 
  impulses, 
  stimula- 
  

   tions, 
  or 
  waves 
  of 
  chemical 
  reaction 
  are 
  always 
  passing 
  from 
  

   the 
  brain 
  of 
  the 
  flying 
  moth 
  to 
  the 
  contracting 
  muscles, 
  and 
  

   if 
  the 
  physiological 
  symmetry 
  of 
  the 
  brain 
  has 
  been 
  disturbed 
  

   by 
  the 
  unequal 
  illumination 
  of 
  the 
  eyes, 
  the 
  muscles 
  on 
  the 
  

   more 
  illumined 
  side 
  are 
  thrown 
  into 
  a 
  state 
  of 
  stronger 
  ten- 
  

   sion 
  or 
  tonus, 
  with 
  the 
  result 
  that 
  they 
  will 
  respond 
  more 
  

   forcibly 
  to 
  stimulation 
  from 
  the 
  brain, 
  and 
  will 
  therefore 
  

   turn 
  the 
  head 
  and 
  body 
  of 
  the 
  moth 
  directly 
  towards 
  the 
  

   candle 
  near 
  which 
  it 
  is 
  flying. 
  " 
  As 
  soon 
  as 
  the 
  plane 
  of 
  

   symmetry 
  goes 
  through 
  the 
  source 
  of 
  light, 
  both 
  eyes 
  receive 
  

  

  