^""FeCisso.^"^"*}  ^^gr^iion  of  Plants  from  Europe  to  America.     1 17 
^tudy  of  the  American  forest  trees,  and  asked  for  an  introduction  to  the  Hickory 
family  of  America,  remarking  that  all  the  members  with  which  he  was  acquainted 
in  Europe  were  fossil  in  the  tertiary  beds  of  his  native  land.  Lastly,  no  fewer  than 
-eight  species  of  smilax,  a  genus  scarcely  known  in  Europe,  but  abundant  in  Amer- 
ica, have  been  found  in  the  Miocene  of  Switzerland. 
We  may  here  remark  in  passing,  that  anyone  desiring  to  see  for  himself  the  close 
resemblance  between  the  European  fossils  and  their  living  American  representatives 
•can  do  so  by  paying  a  visit  to  the  Agassiz  Museum  at  Cambridge,  where,  in  one  of 
the  upper  galleries,  may  be  seen  a  collection  which  has  no  equal  or  second  on  this 
side  of  the  Atlantic. 
It  is  just  necessary  here,  in  or/ier  to  avoid  leaving  a  flaw  in  the  argument,  to  state 
that  many  of  these  species  have  been  discovered  in  beds  of  equal  or  greater  age  in 
this  country.  It  is  therefore  impossible  to  urge  that  they  may  have  passed  from 
Europe  to  America  so  lately  that  changes  have  not  5'et  had  time  to  develop  them- 
selves. On  the  contrary,  some  geologists  are  inclined  to  maintain  that  they  existed 
in  America  before  they  appeared  in  Europe.  At  all  events,  we  are  warranted  in 
asserting  that  during  the  Miocene  age  trees  of  the  kinds  named  grew  in  Europe  and 
America,  as  well  as  in  Greenland  and  Spitzbergen  and  other  points  in  the  far  Ncjrth. 
We  do  not  propose  here  to  investigate  the  causes  of  these  changes.  It  is  suffi- 
cient for  our  purpose  to  maintain  the  fact,  that  during  tertiary  geological  times  the 
European  flora  has  changed,  and  largely  changed,  while  the  American  flora  has 
remained  stationary,  or  nearly  so.  Plants  which  have  changed  in  this  interval  thereby 
•show  an  ability  to  change — a  plasticity — which  may  be  shown  again  should  occa- 
sion arise.  Plants  which  have  not  changed  during  the  same  interval  show  no  proof 
of  possessing  the  same  plasticity.  Moreover,  if  the  principle  is  true  that  long 
•existence  without  change  strengthens  the  habits  or  increases  the  rigidity  of  the 
species,  it  is  a  necessary  inference  that  the  American  flora,  or  so  much  of  it  as  has 
€xisted  during  this  long  interval  unchanged,  must  be  less  plastic  than  the  present 
European  flora,  which  has,  during  the  same  interval,  been  so  largely  modified.  So 
many  ages  of  persistence  in  type  cannot  well  be  without  eff'cct.  Little  as  we  yet 
know  of  geological  time,  we  cannot  estimate  the  age  of  the  Swiss  fossil  plants  at 
less  than  500,000  years,  and  it  may  well  be  twice  as  much.  This  would  place  the 
European  flora  just  as  far  later  or  newer  in  age  and  in  development  than  the  Amer- 
ican— would  give  it  the  advantage  of  so  many  years  of  slow  change — and  may  be 
supposed,  in  some  degree,  to  have  maintained  or  developed  that  plasticity  to  the 
possession  of  which  we  incline  to  attribute  its  ascendency  over  the  native  American 
flora.  On  the  other  hand,  the  native  American  flora,  living  unchanged  through  all 
these  500,000  years,  may  well  have  lost  some  of  the  plasticity  it  perhaps  once  pos- 
sessed, and  have  become  comparatively  rigid,  so  that  it  is  to  that  extent  unable  to 
adapt  itself  suddenly  to  the  changed  condition  of  Europe  at  present.  It  cannot 
therefore  compete  with  the  more  plastic  and  more  highly-developed  forms  which  it 
meets  in  the  Eastern  World;  nor  can  it,  in  all  cases,  even  hold  its  own  against  them 
on  a  soil  and  in  a  climate  where  it  has  dwelt  for  so  many  ages  unmolested.  The 
younger  plant-life  of  Euope,  like  the  white  man,  is  more  than  a  match  for  the  old- 
fashioned  life  of  this  so-called  New  World  of  America,  and  the  weaker  fails  in  the 
