﻿DISTRIBUTION 
  OF 
  FISHES 
  JORDAN 
  359 
  

  

  changes 
  not 
  due 
  in 
  part 
  at 
  least 
  to 
  external 
  causes. 
  The 
  forces 
  behind 
  

   orthogenesis 
  may 
  belong 
  here, 
  but 
  both 
  fact 
  and 
  cause 
  of 
  these 
  phe- 
  

   nomena 
  are 
  still 
  obscure. 
  Another 
  suggestion 
  is 
  that 
  forms 
  become 
  

   "■extinct 
  through 
  exhaustion." 
  This 
  again 
  is 
  purely 
  hypothetical, 
  as 
  

   we 
  know 
  no 
  " 
  exhausted 
  species," 
  although 
  many 
  are 
  very 
  rare, 
  per- 
  

   haps 
  becoming 
  more 
  so. 
  Fresh-water 
  fishes 
  sometimes 
  disappear 
  

   through 
  their 
  inability 
  to 
  meet 
  comi)etition 
  with 
  new 
  arrivals. 
  

   Authors 
  have 
  recognized 
  different 
  types 
  of 
  elimination 
  of 
  species, 
  

   these 
  being 
  theoretical 
  chiefly 
  and 
  not 
  certainly 
  knoAvn 
  to 
  occur 
  in 
  

   nature. 
  

  

  The 
  limits 
  of 
  distribution 
  of 
  individual 
  species 
  or 
  genera 
  must 
  be 
  

   found 
  in 
  some 
  sort 
  of 
  barrier, 
  past 
  or 
  present. 
  The 
  chief 
  barriers 
  

   which 
  limit 
  marine 
  fishes 
  are 
  the 
  presence 
  of 
  land, 
  the 
  presence 
  of 
  

   great 
  oceans, 
  the 
  differences 
  of 
  temperature 
  arising 
  from 
  differences 
  

   in 
  latitude, 
  the 
  nature 
  of 
  the 
  sea 
  bottom, 
  and 
  the 
  direction 
  of 
  the 
  

   oceanic 
  currents. 
  That 
  which 
  is 
  a 
  barrier 
  to 
  one 
  species 
  may 
  be 
  an 
  

   agent 
  in 
  distribution 
  to 
  another. 
  The 
  common 
  shore 
  fishes 
  would 
  

   perish 
  in 
  deep 
  waters 
  almost 
  as 
  surely 
  as 
  on 
  land, 
  while 
  the 
  open 
  

   Pacific 
  is 
  a 
  broad 
  highway 
  to 
  the 
  albacore 
  or 
  the 
  swordfish. 
  

  

  Again, 
  that 
  which 
  is 
  a 
  barrier 
  to 
  rapid 
  distribution 
  may 
  become 
  

   an 
  agent 
  in 
  the 
  slow 
  extension 
  of 
  the 
  range 
  of 
  a 
  species. 
  The 
  great 
  

   Continent 
  of 
  Asia 
  is 
  undoubtedly 
  one 
  of 
  the 
  greatest 
  barriers 
  to 
  the 
  

   wide 
  movement 
  of 
  species 
  of 
  fish, 
  yet 
  its 
  long 
  shore 
  line 
  enables 
  

   species 
  to 
  creep, 
  as 
  it 
  were, 
  from 
  bay 
  to 
  bay, 
  or 
  from 
  rock 
  to 
  rock, 
  

   till, 
  in 
  many 
  cases, 
  the 
  same 
  species 
  is 
  found 
  in 
  the 
  Red 
  Sea 
  and 
  in 
  

   the 
  tide 
  pools 
  or 
  sand 
  reaches 
  of 
  Japan. 
  In 
  the 
  North 
  Pacific 
  the 
  

   presence 
  of 
  a 
  range 
  of 
  half-submerged 
  volcanoes, 
  known 
  as 
  the 
  

   Aleutian 
  and 
  Kurile 
  Islands, 
  has 
  greatly 
  aided 
  the 
  slow 
  movement 
  

   of 
  the 
  fishes 
  of 
  the 
  tide 
  pools 
  and 
  the 
  kelp. 
  To 
  a 
  school 
  of 
  mackerel 
  

   or 
  of 
  flying 
  fishes 
  these 
  rough 
  islands 
  with 
  their 
  narrow 
  channels 
  

   might 
  form 
  an 
  insuperable 
  barrier. 
  

  

  It 
  has 
  long 
  been 
  recognized 
  that 
  the 
  matter 
  of 
  temperature 
  is 
  the 
  

   central 
  fact 
  in 
  all 
  problems 
  of 
  geographical 
  distribution. 
  Few 
  

   species 
  in 
  any 
  group 
  freely 
  cross 
  the 
  frost 
  line, 
  and, 
  except 
  as 
  borne 
  

   by 
  oceanic 
  currents, 
  not 
  many 
  extend 
  their 
  range 
  far 
  into 
  waters 
  

   colder 
  than 
  those 
  in 
  which 
  the 
  species 
  is 
  distinctively 
  at 
  home. 
  

   Knowing 
  the 
  average 
  temperature 
  of 
  the 
  water 
  in 
  a 
  given 
  region, 
  we 
  

   know 
  in 
  general 
  the 
  types 
  of 
  fishes 
  which 
  must 
  inhabit 
  it. 
  It 
  is 
  

   the 
  similarity 
  in 
  temperature 
  and 
  physical 
  conditions 
  which 
  chiefly 
  

   explains 
  the 
  resemblance 
  of 
  the 
  Japanese 
  fauna 
  and 
  that 
  of 
  the 
  

   Mediterranean 
  or 
  the 
  Antilles. 
  This 
  fact 
  alone 
  must 
  explain 
  the 
  

   resemblance 
  of 
  the 
  Arctic 
  and 
  Antarctic 
  faunoe, 
  there 
  being 
  in 
  no 
  

   case 
  a 
  barrier 
  in 
  the 
  sea 
  that 
  may 
  not 
  some 
  time 
  be 
  crossed. 
  Like 
  

   forms 
  lodge 
  in 
  like 
  places. 
  Similarity 
  of 
  conditions 
  produces 
  con- 
  

   vergencies 
  of 
  type 
  but 
  never 
  homology. 
  

  

  