﻿230 
  Panama 
  Shells. 
  introduction. 
  

  

  coast 
  of 
  North 
  America^ 
  a 
  polar 
  current 
  appears 
  to 
  extend 
  

   the 
  Arctic 
  Fauna 
  of 
  marine 
  shells 
  to 
  41° 
  N. 
  lat., 
  and 
  the 
  cold 
  

   current 
  along 
  the 
  shores 
  of 
  the 
  Middle 
  and 
  Southern 
  States 
  

   seems 
  to 
  limit 
  the 
  tropical 
  Fauna 
  to 
  about 
  26° 
  or 
  28° 
  N. 
  lat. 
  

   on 
  the 
  coast, 
  although 
  on 
  the 
  east 
  side 
  of 
  the 
  Grulf 
  Stream 
  it 
  

   extends 
  to 
  the 
  Bermuda 
  Islands 
  in 
  latitude 
  32° 
  N. 
  

  

  It 
  will 
  be 
  seen 
  also 
  in 
  the 
  following 
  pages, 
  that 
  a 
  large 
  

   number 
  of 
  the 
  species 
  which 
  occur 
  at 
  Panama 
  were 
  collected 
  

   by 
  Mr. 
  Cuming 
  at 
  the 
  (3rallapago 
  Islands. 
  But 
  if 
  any 
  of 
  the 
  

   species 
  occur 
  in 
  the 
  Polynesian 
  Islands, 
  the 
  number 
  does 
  not 
  

   exceed 
  three 
  or 
  four, 
  and 
  in 
  these 
  cases 
  the 
  specific 
  identity 
  

   of 
  the 
  shells 
  is 
  very 
  doubtful. 
  The 
  western 
  boundary 
  of 
  the 
  

   Panama 
  province 
  may 
  therefore 
  be 
  made 
  to 
  include 
  only 
  the 
  

   Gallapago 
  Islands. 
  

  

  Thus, 
  in 
  the 
  language 
  of 
  one 
  hypothesis, 
  climate 
  prevents 
  

   the 
  dispersion 
  of 
  the 
  species 
  to 
  the 
  north 
  or 
  south 
  ; 
  and 
  the 
  

   Antarctic 
  current 
  setting 
  over 
  from 
  the 
  coast 
  of 
  Peru 
  to 
  the 
  

   Gallapagos, 
  has 
  favored 
  their 
  dispersion 
  to 
  those 
  islands, 
  and 
  

   the 
  broad 
  sea 
  beyond 
  has 
  restrained 
  them 
  from 
  any 
  farther 
  

   dispersion 
  westward. 
  Perhaps 
  this 
  is 
  as 
  favorable 
  an 
  illus- 
  

   tration 
  as 
  can 
  be 
  found 
  of 
  the 
  doctrine 
  of 
  physical 
  causes 
  ac- 
  

   counting 
  for 
  the 
  distribution 
  of 
  the 
  individuals 
  of 
  each 
  species 
  

   rom 
  a 
  single 
  centre. 
  Yet 
  this 
  doctrine 
  is 
  obviously 
  a 
  mere 
  

   hypothesis. 
  If 
  it 
  be 
  granted 
  that 
  these 
  moUusks 
  can 
  travel 
  

   coastwise 
  until 
  they 
  find 
  the 
  waters 
  too 
  cold 
  for 
  them, 
  it 
  

   would 
  still 
  remain 
  to 
  be 
  proved 
  that 
  they 
  actually 
  do 
  so. 
  

  

  But 
  the 
  supposed 
  dispersion 
  of 
  the 
  species 
  over 
  the 
  sea 
  to 
  

   the 
  G-allapagos, 
  presents 
  more 
  difficulties 
  than 
  at 
  first 
  appear. 
  

   The 
  shells 
  of 
  Cirrhopods 
  which 
  adhere 
  to 
  floating 
  wood, 
  

   have 
  been 
  often 
  found 
  far 
  out 
  at 
  sea 
  ; 
  perhaps, 
  too, 
  some 
  

   species 
  of 
  Pholas 
  may 
  be 
  distributed 
  in 
  tliis 
  manner. 
  But 
  

   the 
  only 
  way 
  in 
  which 
  such 
  dispersion 
  of 
  the 
  species 
  generally 
  

   can 
  be 
  accounted 
  lor 
  is 
  by 
  an 
  imaginary 
  voyage 
  of 
  their 
  spawn 
  

   across 
  the 
  sea. 
  The 
  species, 
  in 
  those 
  stages 
  of 
  growth 
  which 
  

   have 
  been 
  observed, 
  are 
  mostly 
  restricted 
  to 
  narrow, 
  vertical 
  

  

  