1 1 2    Migration  of  Plants  from  Europe  to  America,  { ^""rlCis^a'''"'' 
grow  in  Canada  or  the  States  must,  ere  now,  have  been  many  times  exported  to  the 
mother  country.  Yet  they  do  not  appear.  It  may  be  replied  that  the  greater  pari 
of  the  corn  crops  are  destined  for  the  mill  and  not  for  the  land,  and  that  in  this  way 
their  chances  of  propagation  are  largely  diminished.  Making  all  due  allowance  for 
this,  should  we  not  look  for  a  rank  crop  of  American  weeds  springing  up  around 
the  mills  from  the  cleanings  and  the  waste  ?  Yet  such  is  not  the  case.  With  all 
the  millions  of  bushels,  moreover,  that  go  to  England  for  feeding  purposes,  and  are 
never  ground,  there  is  the  same  result.  The  weeds  no  more  take  root  and  run  wild 
than  do  the  wheat  and  maize  among  which  they  cross  the  Atlantic.  And  when,  in 
addition  to  this,  we  consider  that  there  has  been  for  two  centuries  an  organized  and 
regular  introduction  of  American  wild  plants  into  European  botanic  and  flower 
gardens,  might  we  not  reasonably  expect  to  see  at  least  a  few  of  them,  or  of  others 
which  must  have  accidentally  accompanied  them,  spreading  outside  of  the  limits  of 
these  gardens,  and  becoming  naturalized  in  Europe  ?  Yet  nothing  of  the  kind  has 
occurred.  Neither  the  rank  and  abundant  Ragweed  [Ambrosia)^  nor  the  widely  dif- 
fused Golden  Rods  [Solidago]^  nor  the  Protean  Asters  {Aster),  nor  the  wayside 
Pepper  Grasses  [Lepidium),  nor  the  prolific  Sumachs  [Rhus),  nor  the  clinging  Burr- 
marigolds  [Bidens),  nor  the  ubiquitous  and  striking  Milkweeds  [Asclepias),  have 
succeeded  in  naturalizing  themselves  in  England.  Even  where  a  genus  contains 
species  on  both  sides  of  the  Atlantic,  as  is  the  case  with  the  Houndstongue  [Cyno- 
glossum),  we  find  that  the  English  species — the  common  Houndstongue  [C.  njul- 
gare) — has  migrated  westward,  and  become  so  common  near  Montreal  and  almost 
everywhere  in  the  Eastern  and  Midland  States  that  Professor  Gray  can  term  it  "a 
familiar  and  troublesome  weed  "  j  while  at  the  same  time,  the  common  American 
species,  or  Beggar-lice  Houndstongue  (C.  Morisont),  which  the  same  writer  brands 
as  "a  common  and  vile  weed, '  is  completely  unknown  in  England, 
Some  may  be  inclined  to  urge  that  the  comparatively  cool  English  summer  may 
not  afford  sufficient  heat  to  perfect  and  ripen  the  seed,  which  the  fiercer  sun  and 
continental  climate  of  Eastern  America  can  easily  mature.  This  may  account  for 
the  inability  of  some  American  species  to  sustain  themselves  in  England,  but  it  is 
evidently  far  from  sufficient  to  solve  the  whole  problem.  Many  of  these  plants  can 
perpetuate  themselves  in  the  short,  cool  summer  of  New  England  and  Lower  Can- 
ada, and  we  might  therefore  reasonably  expect,  even  if  want  of  summer  heat 
excluded  them  from  England,  that  they  would  find  a  congenial  climate  somewhere 
in  the  warmer  countries  of  Southern  Europe.  But  not  in  England  only,  but  through- 
out Europe,  the  absence  of  American  species  is  remarkable.  Difference  of  climate 
seems  insufficient  as  the  only  or  the  chief  factor  in  the  solution  of  the  problem,  and 
we  are  compelled  to  look  farther. 
Nor  can  it  be  urged  as  an  objection  that  European  weeds  alone  have  come  in. 
Without  at  present  defining  a  weed,  the  reply  is  obvious  that  American  weeds  have 
not  gained  a  foothold  in  Europe.  It  is  not  to  be  anticipated  that  large,  conspicu- 
ous and  slow-growing  plants,  such  as  forest  trees,  or  highly  developed  and  cultivated 
forms,  such  as  garden- flowers,  will  often  run  wild.  The  former  require  too  long  a 
time  to  grow  and  propagate  themselves,  and  are  subject  to  too  many  dangers,  while 
the  latter  are  only  maintained  at  their  high  standing  by  constant  and  careful  culti- 
