114    Migration  of  Plants  from  Europe  to  America.  {^"'VebirisaJT'™' 
plastic  nature — one  capable  of  being  moulded  by  and  to  its  new  surroundings — ere 
long  adapts  itself,  if  the  change  is  not  too  great  or  sudden,  to  its  new  situation,  takes 
out  a  new  lease  of  life,  and  continues  in  the  strictest  sense  a  nxjeed. 
Is  it  not  possible  that  some  such  cause  as  this  may  lie  underneath  the  facts  we 
detailed  in  the  earlier  part  of  this  paper  ?  The  true  and  full  explanation  of  the 
transfer  of  European  species  to  America  should  at  the  same  time  explain  the  absence 
of  American  species  from  Europe.  But  the  partial  causes  already  alluded  to  fail  to 
do  this.  There  is  a  residual  elfect  for  which  they  do  not  account.  May  it  not  be 
true  that  the  plants  of  the  European  flora  possess  more  of  this  plasticity,  are  less 
unyielding  in  their  constitution,  can  adapt  themselves  more  readily  to  their  surround- 
ings, and  thus  secure  their  continuance  in  the  New  World  ?  And  may  it  not  be  the 
lack  of  this  plasticity  in  the  American  flora  which  incapacitate?  it  for  securing  a 
foothold  and  obtaining  a  living  in  the  different  conditions  of  the  Old  World  ?  Under 
the  care  of  the  gardener  they  grow  and  embellish  the  gardens  and  conservatories  of 
Europe,  but  without  this  care  they  speedily  fail  and  die. 
To  point  out  the  physiological  basis  of  this  property  of  plasticity  is  at  presenr 
and  will  probably  long  remain  impossible.  But  that  such  a  property  exists  in  both 
the  animal  and  vegetable  kingdoms  is  beyond  a  question.  It  is  the  secret  of  that 
variation  which  so  strongly  marks  some  species,  while  its  absence  is  the  cause  of 
that  fixedness  which  characterizes  others.  It  is  the  secret  of  that  quick  response 
wliich  some  plants  make  to  a  change  of  conditions,  and  whereby  they  gain  fresh 
vitality  at  the  cost,  it  may  be,  of  some  slight  modification  of  structure.  Its  absence, 
on  the  contrary,  causes  that  indifference  or  resistance  which  characterizes  others, 
and  which  is  almost  always  followed  sooner  or  later  by  the  extinction  of  the  resist- 
ing species. 
Though,  however,  the  indication  of  the  exact  physiological  basis  of  this  plasticity 
of  constitution  is  as  yet  beyond  our  reach,  it  seems  possible  to  point  out  one  fact 
which  not  improbably  has  had  some  share  in  reducing  the  plasticity  of  the  American 
flora.  To  approach  a  single  short  step  nearer  to  the  object  of  our  quest,  when  that 
object  is  at  present  unattainable,  is  so  much  ground  gained  We  are  all  familiar 
with  the  effects  of  habit  upon  ourselves.  We  all  know  how  easy  habitual  actions 
become  ;  how  strong  is  the  tendency  to  perform  them  when  the  conditions  recur 
under  which  they  are  usually  performed,  and  how  unwillingly  we  deviate  from  our 
daily  course  after  following  it  for  years.  To  this  one  fact — the  power  of  habit — is 
due  the  uncomfortable,  unsettled  state  of  most  men  who  make  some  great  change 
in  their  outward  surroundings  late  in  life.  Few  who  emigrate  in  old  age  ever  become 
quite  reconciled  to  their  new  home.  The  habits  of  many  years  have  so  moulded 
them  in  body  and  mind,  and  set  them  so  firmly  in  their  mould,  that  the  plasticity 
they  may  have  once  possessed  is  gone,  as  bricks  dried  and  burnt  have  lost  the 
pliancy  they  possessed  when  in  the  form  of  clay.  Of  the  physiological  cause  of 
this  fact  we  know  nothing,  but  the  fact  no  one  can  doubt.  Experience  shows  us 
that  habit  is  no  less  powerful  in  plants  than  in  animals.  What  a  plant  has  been  in 
the  habit  of  doing  that  it  will  incline  to  do  again.  The  physical  organization  of 
the  plant,  acted  upon  by  the  conditions  that  surround  it,  produces  its  habit.  The 
longer  these  remain  unchanged  the  longer  do  its  habits  continue,  and  the  longer  its 
