^"'  Feb.ri88o"°''}  Migration  of  Plants  from  Europe  to  America.    1 1 1 
another  side  of  the  question  must  not  be  overlooked.  Many  common  English 
plants  have  totally  failed  to  secure  a  foothold  here.  The  seed  of  the  English  daisy 
must  have  come  over  in  almost  every  case  of  grass-seed  that  has  been  imported; 
yet  it  has  not  become  naturalized  in  America  The  only  instance  with  which  the 
writer  is  acquainted  of  its  lasting  for  several  years  in  a  lawn,  as  in  England,  occurred 
in  the  immediate  vicinity  of  Montreal,  and  was  communicated  to  him  by  a  friend  in 
that  city.  It  would  therefore  appear  that  the  heat  of  summer  rather  than  the  cold 
of  winter  is  the  barrier  to  the  establishment  in  the  New  World  of  the  "wee  modest 
crimson-tipped  flower,"  so  familiar  to  every  British  eye.  Equally  "  conspicuous  by 
their  absence"  are  the  Primrose  and  the  Cowslip,  the  flowers  of  childhood  in  the 
old  country.  Many  other  instances  of  this  kind  might  be  adduced,  but  the  con- 
verse side  of  the  problem  now  claims  attention. 
It  is  singular  that  while  so  many  European  species  have  forced  their  way  into 
possession  of  the  American  soil,  the  cases  of  counter  migration  are  exceedingly 
few^ — so  few  that  they  may  be  counted  on  the  fingers.  It  appears  as  if  some  invis- 
ible barrier  existed  preventing  passage  eastward,  though  allowing  it  westward.  One 
or  two  species  may  be  named  which,  as  exceptions,  bring  the  general  truth  of  this 
statement  into  stronger  light.  The  Canadian  Fleabane  [Erigeron  Canadensis)^  a 
native  of  North  America,  "is  now  established  in  nearly  all  temperate  and  hot  coun- 
tries, and  occasionally  appears  so  in  England."  ^  The  Annual  Fleabane  [Erigeron 
annuus),  though  not  in  England,  has  become  wild  in  some  parts  of  Europe.  Add 
to  these  two  the  so-called  "  Water  Thyme"  {Elodea  Canadensis)^  and  we  have  all 
the  conspicuous  examples  with  which  the  writer  is  acquainted  of  the  eastward  migra- 
tion of  American  plants  to  Europe  and  their  naturalization  there  The  last  named 
plant  was  first  observed  about  1847  in  the  northern  and  midland  counties  of  Eng- 
land and  the  south  of  Scotland,  in  Yorkshire,  Leicestershire  and  near  Berwick  and 
Edinburgh.  2  How  it  was  introduced  is  not  known.  Thence  it  spread  until  in 
about  ten  years  many  of  the  slower  streams  were  almost  clogged  with  it,  and  the 
writer  well  recollects  that  it  was  then  difficult  to  row  on  the  upper  and  middle 
Thames  in  consequence  of  the  accumulation  of  this  weed.  Fears  were  even  enter- 
tained that  it  would  form  a  serious  impediment  to  inland  navigation.  But  in  a 
short  time  the  evil  diminished,  and  after  a  few  years,  though  still  present,  the  quan- 
tity in  the  rivers  became  insignificant,  and  no  inconvenience  is  now  caused  by  its 
presence. 
Such  facts  naturally  suggest  the  question:  Why  are  these  things  so.?  What 
invisible  door  bars  the  passage  of  the  American  flora  to  Europe,  but  admits  the  free 
passage  of  the  European  flora  to  America?  One  reply  will  naturally  occur.  Seed 
is  mainly  brought  from  Europe  to  America,  and  thereby  a  favorable  chance  is 
afforded  for  introducing  the  seeds  of  European  weeds.  This  is  so ;  and  to  this 
cause,  doubtless,  is  due  the  immense  number  of  introduced  plants.  But,  if  Euro- 
pean seed  is  largely  brought  to  America,  American  crops  go  much  more  largely  to 
Europe;  and  it  would  be  absurd  to  suppose  that  any  crop  gathered  from  the  half- 
cleared  and  weedy  fields  of  this  country  could  be  sent  thither  without,  at  the  same 
time  sending  in  abundance  the  seeds  of  our  native  weeds.    All  the  ill  weeds  that 
Bentham's  "  Handbook  of  the  British  Flora." 
2  Bid. 
