﻿134 
  niOCEEDINGS 
  OF 
  THE 
  MALACOLOGICAL 
  SOCIETY. 
  

  

  of 
  Mexico 
  and 
  Central 
  America, 
  would 
  seem 
  to 
  be 
  an 
  argument 
  

   against 
  that 
  view. 
  But 
  however 
  that 
  may 
  be, 
  in 
  the 
  absence 
  of 
  

   any 
  direct 
  evidence 
  on 
  the 
  subject, 
  it 
  might 
  be 
  said 
  that 
  it 
  is 
  

   quite 
  as 
  possible 
  that 
  the 
  genus 
  sprang 
  from 
  the 
  series 
  of 
  mutations 
  

   involved 
  in 
  the 
  evolutionary 
  history 
  of 
  the 
  later 
  Cretaceous 
  

   TJnione 
  fauna 
  of 
  Europe 
  and 
  Asia, 
  which 
  from 
  some 
  cause 
  became 
  

   extinct 
  in 
  Europe, 
  but 
  which, 
  under 
  more 
  favourable 
  conditions 
  of 
  

   environment, 
  survived 
  in 
  Asia 
  and 
  gave 
  rise 
  to 
  the 
  present 
  fauna 
  

   of 
  that 
  country, 
  as 
  that 
  it 
  originated 
  in 
  the 
  Cretaceous 
  mutations 
  of 
  

   the 
  North. 
  American 
  fauna. 
  The 
  existence 
  of 
  a 
  similar 
  species 
  

   of 
  the 
  genus 
  (Jf. 
  Laosensis) 
  at 
  the 
  present 
  time 
  in 
  Siam 
  and 
  

   Burmah, 
  would 
  also 
  seem 
  to 
  be 
  of 
  considerable 
  significance 
  as 
  

   bearing 
  on 
  the 
  probability 
  of 
  an 
  Asiatic 
  origin 
  of 
  the 
  genus. 
  

   In 
  view 
  of 
  the 
  remarkable 
  tenacity 
  of 
  M. 
  margaritifera 
  in 
  retaining 
  

   its 
  specifi-C 
  characteristics, 
  it 
  is 
  evident 
  that 
  the 
  separation 
  of 
  the 
  

   Siamese 
  race 
  must 
  have 
  taken 
  place 
  at 
  a 
  remote 
  period. 
  

  

  The 
  conclusion 
  of 
  the 
  whole 
  matter, 
  so 
  far 
  as 
  the 
  question 
  of 
  the 
  

   origin 
  of 
  the 
  genus 
  is 
  concerned, 
  would, 
  therefore, 
  seem 
  to 
  be 
  this 
  : 
  

   There 
  is 
  no 
  direct 
  evidence 
  available 
  at 
  the 
  present 
  proving 
  either 
  

   that 
  it 
  was 
  American 
  or 
  Asiatic. 
  So 
  that, 
  for 
  the 
  time 
  being, 
  the 
  

   decision 
  must 
  be 
  based 
  upon 
  such 
  legitimate 
  deductions 
  as 
  can 
  be 
  

   drawn 
  from 
  the 
  known 
  facts 
  of 
  present 
  distribution, 
  taken 
  in 
  con- 
  

   nexion 
  with 
  such 
  recognized 
  palseogeographic 
  changes 
  in 
  continental 
  

   areas 
  as 
  would 
  seem 
  to 
  substantiate 
  the 
  theory 
  proposed. 
  If 
  an 
  

   Asiatic 
  centre 
  of 
  dispersal 
  be 
  assumed, 
  for 
  purposes 
  of 
  argument, 
  

   and 
  a 
  subsequent 
  western 
  migration 
  through 
  northern 
  Europe 
  and 
  

   across 
  the 
  Greenland 
  bridge 
  in 
  Miocene 
  or 
  early 
  Pliocene 
  times 
  

   (Scharff, 
  loc. 
  cit., 
  p. 
  123), 
  there 
  is 
  at 
  once 
  at 
  hand 
  a 
  reasonable 
  

   explanation 
  for 
  the 
  peculiar 
  distribution 
  of 
  the 
  species 
  in 
  eastern 
  

   North 
  America. 
  Whether 
  this 
  intercontinental 
  migration 
  was 
  

   eastward 
  or 
  westward, 
  it 
  must 
  have 
  taken 
  place 
  at 
  this 
  period. 
  

   Not 
  only 
  it 
  could 
  not 
  have 
  occurred 
  later, 
  but 
  it 
  must 
  have 
  been 
  

   sufficiently 
  prior 
  to 
  the 
  Glacial 
  Period 
  to 
  allow 
  the 
  species 
  to 
  proceed 
  

   far 
  enough 
  south 
  along 
  the 
  American 
  coast 
  to 
  allow 
  a 
  remnant 
  to 
  

   survive 
  the 
  ice-sheet, 
  and 
  to 
  serve 
  as 
  a 
  new 
  centre 
  of 
  dispersal 
  upon 
  

   its 
  retirement. 
  

  

  Whether 
  this 
  original 
  immigration, 
  if 
  it 
  came 
  from 
  the 
  east, 
  succeeded 
  

   in 
  reaching 
  central 
  British 
  America, 
  is 
  not 
  known. 
  If 
  it 
  did, 
  it 
  was 
  

   no 
  doubt 
  wholly 
  exterminated 
  by 
  the 
  ice 
  in 
  that 
  region. 
  That 
  it 
  was 
  

   entirely 
  wiped 
  out 
  at 
  this 
  time 
  by 
  the 
  glaciers 
  along 
  the 
  sea-coast 
  is 
  

   not 
  so 
  clear. 
  There 
  is 
  apparently 
  reason 
  to 
  believe 
  that 
  a 
  considerable 
  

   portion 
  of 
  the 
  fauna 
  survived 
  through 
  the 
  Glacial 
  Period 
  as 
  far 
  north 
  as 
  

   Greenland 
  (Scharff, 
  loc. 
  cit., 
  p. 
  1 
  23). 
  And 
  the 
  fact 
  that 
  M. 
  margaritifera 
  

   is 
  at 
  the 
  present 
  time 
  found 
  on 
  both 
  Newfoundland 
  and 
  Anticosti 
  

   would 
  seem 
  to 
  show 
  that 
  on 
  both 
  of 
  these 
  islands 
  it 
  was, 
  by 
  favourable 
  

   environmental 
  conditions, 
  enabled 
  to 
  survive 
  during 
  that 
  period. 
  And 
  

   it 
  is 
  quite 
  possible 
  that 
  similar 
  conditions 
  may 
  have 
  enabled 
  isolated 
  

   colonies 
  to 
  survive 
  on 
  the 
  coast 
  of 
  the 
  mainland. 
  There 
  is 
  also 
  reason 
  

   to 
  believe 
  that 
  a 
  similar 
  survival 
  of 
  the 
  fauna 
  in 
  favourable 
  locations 
  

   during 
  the 
  Glacial 
  Period 
  may 
  have 
  occurred 
  on 
  the 
  Pacific 
  coast 
  (see 
  

  

  