﻿Double 
  Adaptations 
  453 
  

  

  adaptation, 
  and 
  on 
  such 
  a 
  base 
  a 
  hypothetical 
  

   explanation 
  could 
  no 
  doubt 
  be 
  rested. 
  

  

  Instead 
  of 
  discussing 
  this 
  problem 
  from 
  the 
  

   theoretical 
  side, 
  I 
  prefer 
  to 
  compare 
  those 
  spe- 
  

   cies 
  which 
  are 
  capable 
  of 
  assuming 
  a 
  dwarf 
  

   stature 
  under 
  less 
  uncommon 
  conditions 
  than 
  

   those 
  of 
  alpine 
  and 
  desert-plants. 
  Many 
  weeds 
  

   of 
  our 
  gardens 
  and 
  many 
  wild 
  species 
  have 
  this 
  

   capacity. 
  They 
  become 
  very 
  tall, 
  with 
  large 
  

   leaves, 
  richly 
  branched 
  sterns 
  and 
  numerous 
  

   flowers 
  in 
  moist 
  and 
  rich 
  soil. 
  On 
  bad 
  soil, 
  or 
  

   if 
  germinating 
  too 
  late, 
  when 
  the 
  season 
  is 
  

   drier, 
  they 
  remain 
  very 
  small, 
  producing 
  only 
  

   a 
  few 
  leaves 
  and 
  often 
  limiting 
  themselves 
  to 
  

   one 
  flower-head. 
  This 
  is 
  often 
  seen 
  with 
  thorn- 
  

   apples 
  and 
  amaranths, 
  and 
  even 
  with 
  oats 
  and 
  

   rye, 
  and 
  is 
  notoriously 
  the 
  case 
  with 
  buck- 
  

   wheat. 
  G-a.uchery 
  has 
  observed 
  that 
  the 
  ex- 
  

   tremes 
  differ 
  often 
  as 
  much 
  from 
  one 
  another 
  

   as 
  1 
  :10. 
  In 
  the 
  case 
  of 
  the 
  Canadian 
  horse- 
  

   weed 
  or 
  Erigeron 
  canade.nsis 
  , 
  which 
  is 
  widely 
  

   naturalized 
  in 
  Europe, 
  the 
  tallest 
  specimens 
  

   are 
  often 
  twenty-five 
  times 
  as 
  tall 
  as 
  the 
  small- 
  

   est, 
  the 
  difference 
  increasing 
  to 
  greater 
  ex- 
  

   tremes, 
  if 
  besides 
  the 
  main 
  stern, 
  the 
  length 
  of 
  

   the 
  numerous 
  branches 
  of 
  the 
  tall 
  plants 
  are 
  

   taken 
  into 
  consideration. 
  Other 
  instances 
  

   studied 
  by 
  the 
  French 
  investigator 
  are 
  Ery- 
  

   tliraea 
  pulchella 
  and 
  Calamintha 
  Acinos. 
  

  

  