﻿§32 
  Panama 
  Shells'. 
  introduction 
  

  

  Fissureila 
  ni^ropuhciata 
  ; 
  on 
  stones 
  and 
  rocks, 
  at 
  and 
  be- 
  

   low 
  half 
  tide 
  level. 
  

  

  Siphonaria 
  gig'as 
  ; 
  on 
  rooks 
  near 
  half 
  tide 
  level. 
  

  

  Thus 
  it 
  appears 
  that 
  a 
  larger 
  proportion 
  of 
  the 
  littoral 
  

   than 
  of 
  the 
  pelagic 
  knov;n 
  species 
  are 
  common 
  to 
  the 
  Galla- 
  

   pago 
  Islands 
  and 
  the 
  mainland 
  : 
  and 
  of 
  the 
  pelagic 
  species, 
  

   all 
  v.'hich 
  are 
  known 
  inhabit 
  moderate 
  depths. 
  Locomotion, 
  

   therefore, 
  beneath 
  the 
  sea 
  cannot 
  be 
  assumed 
  as 
  a 
  probable 
  

   means 
  of 
  dispersion. 
  

  

  If, 
  however, 
  the 
  distribution 
  is 
  due 
  to 
  dispersion 
  by 
  the 
  

   floating 
  of 
  the 
  spawn 
  westward, 
  along 
  with 
  the 
  prevailing 
  

   winds 
  and 
  currents, 
  it 
  is 
  remarkable 
  that 
  there 
  are 
  no 
  well 
  

   authenticated 
  examples 
  of 
  clearly 
  identical 
  species 
  which 
  are 
  

   common 
  to 
  the 
  G-allapagos 
  and 
  to 
  the 
  Polynesian 
  Islands. 
  

   In 
  respect 
  of 
  species, 
  which 
  so 
  nearly 
  resemble 
  each 
  other 
  as 
  

   to 
  be 
  sometimes 
  confounded, 
  and 
  which 
  are 
  by 
  some 
  authors 
  

   regarded 
  as 
  varieties 
  produced 
  by 
  local 
  conditions, 
  the 
  V/est 
  

   Indies 
  furnish 
  as 
  man}" 
  of 
  these 
  analogues 
  to 
  the 
  Panama 
  

   species 
  as 
  have 
  been 
  found 
  in 
  Polynesia. 
  There 
  is, 
  therefore, 
  

   no 
  reason 
  for 
  referring 
  the 
  Polynesian 
  analogues 
  to 
  a 
  conti- 
  

   nental 
  origin. 
  

  

  If 
  suck 
  dispersion 
  westward 
  were 
  a 
  fact, 
  then 
  we 
  ought 
  

   to 
  iind, 
  as 
  we 
  go 
  westward 
  from 
  the 
  Pacific 
  shores 
  of 
  America, 
  

   a 
  continual 
  accumulation 
  of 
  species. 
  For, 
  in 
  addition 
  to 
  the 
  

   full 
  complement 
  of 
  aboriginal 
  species 
  in 
  any 
  region, 
  there 
  

   Would 
  be 
  a 
  per 
  centage 
  of 
  immigrant 
  species. 
  If 
  such 
  dis- 
  

   persion 
  were 
  a 
  common 
  fact, 
  the 
  Gfallapagos 
  should 
  be 
  much 
  

   richer 
  in 
  species 
  than 
  the 
  continent, 
  and 
  the 
  v.^estern 
  regions 
  

   of 
  Polynesia 
  and 
  the 
  Australasian 
  Archipelago 
  should 
  contain 
  

   a 
  great 
  number 
  of 
  immigrant 
  species 
  in 
  addition 
  to 
  their 
  ab- 
  

   original 
  Fauna:. 
  But 
  there 
  are 
  "no 
  facts 
  which 
  correspond 
  

   with 
  such 
  hypotheses. 
  So 
  far 
  as 
  is 
  known, 
  the 
  Bay 
  of 
  Panama 
  

   is 
  as 
  rich 
  in 
  species 
  as 
  any 
  region 
  westward, 
  with 
  no 
  more 
  

  

  