^^'^FeCis^s^!"'"'"}  Migration  of  Plants  from  Europe  to  America,     i  i  5 
habits  continue  the  more  firmly,  we  must  infer,  do  they  become  ingrained  in  its 
physiological  structure.  Thus  do  habit  and  organization  act  and  react  on  each 
other.  Each  may  be  changed,  but  all  such  changes  are  slow,  and  we  may  easily,  in 
view  of  these  facts,  believe  that  after  many  years  or  ages  of  unchanged  conditions  a 
plant  may  become  (as  many  an  old  man  becomes)  so  firmly  set  in  its  habits,  so  rigid 
in  its  nature,  as  to  resist  modifying  influences  with  all  the  energy  it  possesses,  and 
rather  die  than  change.  This  is  what  we  mean  by  losing  its  plasticity.  A  plant 
accustomed  in  the  climate  of  England  to  occupy  two  months  in  perfecting  its  seed 
may,  if  suddenly  removed  to  another  country,  continue  its  former  practice  or  it  may 
not.  In  the  former  case,  if  the  new  climate  does  not  afford  the  time  required,  the 
seed  is  not  ripened  and  the  species  fails.  If,  however,  the  plant  can  adapt  itself  to 
the  shorter  season,  and  ripen  its  seed  earlier,  it  may  survive.  But  for  this  result  a 
high  degree  of  plasticity  is  needed.  On  the  other  hand,  if  the  change  of  climate  be 
made  more  slowly,  the  habits  and  organization  of  the  plant  may  keep  pace  with  it, 
and  with  even  less  plasticity  than  in  the  former  case,  the  species  may  survive. 
We  may  advance  at  least  one  step  farther.  If  these  views  on  the  relation  of  habit 
and  organization  to  time  be  correct,  have  we  not  a  possible,  though  at  present  a 
rude,  gauge  for  both?  If  the  strength  of  habit  increases  with  time,  may  we  not 
roughly  measure  that  strength  by  the  length  of  time  during  which  the  habit  has 
prevailed?  And  further,  if  the  plasticity  of  plant  nature  diminishes  and  its  rigidity 
increases  with  the  duration  of  a  habit,  may  not  this  duration  in  like  manner  be 
employed  to  some  extent  as  a  gauge  of  rigidity;  that  is  of  want  of  plasticity?  We 
stand  here  on  new  and  difficult  ground,  and  any  deduction  must  be  tested  severely 
before  reliance  can  be  placed  upon  it.  The  confines  of  geology  and  botany,  the 
place  where  the  two  sciences  march  together,  is  almost  unknown  territory  over 
which  science  is  just  beginning  to  extend  its  conquests.  The  tracing  of  earth's 
existing  flowers  into  her  past,  the  genealogy  of  plants,  is  a  subject  closely  connected 
with  that  other  subject — the  descent  of  species — which  now  so  sorely  divides  the 
leaders  in  natural  science.  Nevertheless,  we  propose  in  the  concluding  portion  of 
this  paper  to  grope  out  into  this  unknown  land  where  the  light  is  so  dim,  and  try  to 
feel  our  way  along  the  clue  indicated  above,  in  the  hope  of  finding  some  link  that 
may  connect  the  apparently  inconsistent  facts  we  are  attempting  to  reconcile — the 
abundant  westward  migration  of  plants  from  Europe,  and  their  scanty  eastward 
migration  from  America. 
Have  we  then,  at  the  outset,  any  reason  to  believe  that  the  North  American  flora 
possesses  less  plasticity  than  the  European?  Let  us  apply  the  gauge  just  mentioned 
and  see  the  result — the  gauge  of  time.  We  are  in  the  habit  of  calling  America  the 
**New  World."  Botanically,  and  also,  we  may  add,  zoologically  speaking,  Amer- 
ica is  the  older  and  Europe  the  younger.  Europe  passed  ages  ago  through  the 
stages  of  plant  life  which  America  exhibits  to-day.  The  trees  and  plants  of  Amer- 
ica, like  most  of  her  native  animals,  belong  to  old-fashioned,  antiquated  types — 
types  that  have  passed  away  from  European  life  and  now  lie  entombed  beneath  its 
surface  in  he  records  of  geology.  If  we  turn  for  a  moment  and  consult  these 
buried  registers  of  births  and  deaths,  we  find  that  in  ages  past  the  existing  families 
of  America  were  living  in  Europe.  Name  after  name  may  be  turned  up,  long 
struggle.  Our  country  swarms  with  the  weeds  of  Europe,  while  our  own  weeds 
shrink  from  the  conflict  both  in  Europe  and  at  home.' 
