﻿Double 
  Adaptations 
  449 
  

  

  chance 
  for 
  a 
  sudden 
  change 
  in 
  the 
  desired 
  di- 
  

   rection, 
  as 
  well 
  as 
  for 
  the 
  slow 
  accumulation 
  of 
  

   slight 
  deviations. 
  Any 
  mutations 
  in 
  a 
  wrong 
  

   direction 
  would 
  at 
  once 
  be 
  destroyed, 
  but 
  an 
  

   accidental 
  change 
  in 
  a 
  useful 
  way 
  would 
  be 
  

   preserved, 
  and 
  multiply 
  itself. 
  If 
  in 
  the 
  course 
  

   of 
  centuries 
  this 
  occurred, 
  they 
  would 
  be 
  near- 
  

   ly 
  sure 
  to 
  become 
  established, 
  however 
  rare 
  at 
  

   the 
  outset. 
  Hence 
  the 
  positive 
  assertion 
  is 
  

   scarcely 
  capable 
  of 
  direct 
  proof. 
  

  

  On 
  the 
  other 
  hand 
  the 
  negative 
  assertion 
  

   must 
  be 
  granted 
  full 
  significance. 
  If 
  the 
  al- 
  

   pine 
  climate 
  has 
  done 
  no 
  more 
  than 
  produce 
  a 
  

   transitory 
  change, 
  it 
  is 
  clear 
  that 
  thousands 
  

   of 
  years 
  do 
  not, 
  necessarily, 
  cause 
  constant 
  

   and 
  specific 
  alterations. 
  This 
  requirement 
  is 
  

   one 
  of 
  the 
  indispensable 
  supports 
  of 
  the 
  La- 
  

   marckian 
  theory. 
  The 
  matter 
  is 
  capable 
  of 
  

   disproof 
  however, 
  and 
  such 
  disproof 
  seems 
  to 
  

   be 
  afforded 
  by 
  the 
  direct 
  evidence 
  of 
  the 
  pres- 
  

   ent 
  condition 
  of 
  the 
  alpine 
  varieties 
  at 
  large, 
  

   and 
  by 
  many 
  other 
  similar 
  cases. 
  

  

  Among 
  these 
  the 
  observations 
  of 
  Holter- 
  

   mann 
  on 
  some 
  desert-plants 
  of 
  Ceylon 
  are 
  of 
  

   the 
  highest 
  value. 
  Moreover 
  they 
  touch 
  ques- 
  

   tions 
  which 
  are 
  of 
  wide 
  importance 
  for 
  the 
  

   study 
  of 
  the 
  biology 
  of 
  American 
  deserts. 
  For 
  

   this 
  reason 
  I 
  mav 
  be 
  allowed 
  to 
  introduce 
  them 
  

  

  / 
  

  

  here 
  at 
  some 
  length. 
  

  

  