﻿ANIMAL 
  BEHAVIOUR 
  193 
  

  

  again 
  equal 
  illumination, 
  the 
  tension 
  (or 
  tonus) 
  of 
  sym- 
  

   metrical 
  muscles 
  becomes 
  equal 
  again, 
  and 
  the 
  impulses 
  for 
  

   locomotion 
  will 
  now 
  produce 
  equal 
  activity 
  in 
  the 
  symmetri- 
  

   cal 
  muscles. 
  As 
  a 
  consequence, 
  the 
  animal 
  will 
  move 
  in 
  a 
  

   straight 
  line 
  to 
  the 
  source 
  of 
  light 
  until 
  some 
  other 
  asym- 
  

   metrical 
  disturbance 
  once 
  more 
  changes 
  the 
  direction 
  of 
  mo- 
  

   tion 
  ' 
  (Loeb, 
  1918, 
  p. 
  14). 
  Such, 
  in 
  outline, 
  is 
  Prof. 
  

   Jacques 
  Loeb's 
  ingenious 
  and 
  convincing 
  theory 
  of 
  the 
  tro- 
  

   pism 
  or 
  ' 
  forced 
  movement 
  ' 
  which 
  brings 
  the 
  moth 
  into 
  the 
  

   candle. 
  

  

  Tropistic 
  actions 
  are 
  obligatory 
  in 
  the 
  sense 
  that 
  every 
  

   creature 
  of 
  the 
  same 
  kind 
  and 
  in 
  the 
  same 
  physiological 
  

   state 
  will 
  in 
  similar 
  circumstances 
  behave 
  in 
  the 
  same 
  way 
  ; 
  

   there 
  is 
  no 
  alternative. 
  But 
  it 
  is 
  a 
  very 
  notable 
  fact, 
  to 
  

   be 
  carefully 
  thought 
  over, 
  that 
  a 
  tropism 
  may 
  be 
  changed, 
  

   reversed, 
  or 
  annulled 
  by 
  changes 
  in 
  the 
  physiological 
  con- 
  

   dition 
  of 
  the 
  body 
  or 
  by 
  changes 
  in 
  the 
  surrounding 
  medium. 
  

   The 
  common 
  Amphipod 
  Crustacean 
  Gammarus 
  of 
  fresh- 
  

   water 
  pools 
  always 
  moves 
  away 
  from 
  light 
  that 
  is 
  its 
  tro- 
  

   pism, 
  but 
  add 
  the 
  least 
  trace 
  of 
  acid 
  to 
  the 
  water, 
  and 
  it 
  

   moves 
  towards 
  the 
  light 
  as 
  if 
  a 
  drop 
  of 
  philtre 
  changed 
  

   the 
  creature's 
  whole 
  nature 
  ! 
  

  

  The 
  tropistic 
  movements 
  often 
  appear 
  as 
  if 
  they 
  had 
  a 
  very 
  

   definite 
  external 
  aim 
  such 
  as 
  the 
  candle 
  but 
  that 
  is 
  illu- 
  

   sory. 
  The 
  orientation 
  is 
  physiologically 
  coerced. 
  There 
  is 
  no 
  

   desire 
  of 
  the 
  moth 
  for 
  the 
  star. 
  It 
  should 
  be 
  noted 
  that 
  

   their 
  general 
  adaptiveness 
  is 
  not 
  contradicted 
  by 
  cases 
  like 
  

   the 
  moth 
  flying 
  into 
  the 
  candle, 
  for 
  organisms 
  are 
  not 
  and 
  

   could 
  not 
  be 
  adapted 
  to 
  the 
  altogether 
  exceptional 
  and 
  un- 
  

   natural. 
  In 
  some 
  cases 
  one 
  tropism 
  way 
  thwart 
  another, 
  

   and 
  it 
  may 
  be 
  that 
  a 
  tropistic 
  movement 
  is 
  sometimes 
  in- 
  

   terrupted 
  by 
  some 
  strong 
  internal 
  stimulus 
  such 
  as 
  a 
  desire. 
  

  

  