﻿DISTEIBUTION" 
  OF 
  FISHES 
  — 
  JORDAN 
  

  

  375 
  

  

  unlikely 
  that 
  much 
  of 
  the 
  close 
  resemblance 
  of 
  the 
  fresh-water 
  

   faunae 
  of 
  northern 
  Europe, 
  Asia, 
  and 
  North 
  America 
  is 
  due 
  to 
  this 
  

   fact. 
  To 
  attempt 
  to 
  decide 
  from 
  which 
  side 
  the 
  first 
  migration 
  

   came 
  in 
  regard 
  to 
  each 
  group 
  of 
  fishes 
  might 
  be 
  interesting; 
  but 
  

   without 
  a 
  wider 
  range 
  of 
  facts 
  than 
  is 
  now 
  in 
  our 
  possession, 
  most 
  

   such 
  attempts, 
  based 
  on 
  guesswork, 
  have 
  little 
  value. 
  The 
  inter- 
  

   locking 
  of 
  the 
  fish 
  faunas 
  of 
  Asia 
  and 
  North 
  America 
  presents, 
  

   however, 
  a 
  number 
  of 
  interesting 
  problems, 
  for 
  migrations 
  in 
  both 
  

   directions 
  have 
  doubtless 
  taken 
  place. 
  

  

  One 
  might 
  go 
  on 
  indefinitely 
  with 
  the 
  discussion 
  of 
  special 
  cases, 
  

   each 
  more 
  or 
  less 
  interesting 
  or 
  suggestive 
  in 
  itself, 
  but 
  the 
  general 
  

   conclusion 
  in 
  all 
  cases 
  is 
  the 
  same. 
  The 
  present 
  distribution 
  of 
  

   fishes 
  is 
  the 
  result 
  of 
  the 
  long-continued 
  action 
  of 
  forces 
  still 
  in 
  

   operation. 
  The 
  species 
  have 
  entered 
  our 
  waters 
  in 
  many 
  invasions 
  

   from 
  the 
  Old 
  World 
  or 
  from 
  the 
  sea. 
  Each 
  species 
  has 
  been 
  sub- 
  

  

  

  

  '"*^. 
  .■ 
  ■ 
  i 
  

  

  Fig. 
  6. 
  — 
  Rainbow 
  Trout, 
  Salmo 
  irideus 
  Gibbons, 
  male. 
  Introduced 
  to 
  Vernia, 
  Mo. 
  

  

  jected 
  to 
  the 
  various 
  influences 
  implied 
  in 
  the 
  term 
  "natural 
  selec- 
  

   tion," 
  and 
  also 
  to 
  concurrent 
  effects 
  of 
  isolation 
  and 
  segregation, 
  and 
  

   under 
  varying 
  conditions 
  its 
  representatives 
  have 
  undergone 
  many 
  

   different 
  modifications. 
  Each 
  of 
  the 
  681 
  fresh-water 
  species 
  we 
  now 
  

   know 
  in 
  the 
  United 
  States 
  may 
  be 
  conceived 
  as 
  making 
  every 
  year 
  

   inroads 
  on 
  territory 
  occupied 
  by 
  other 
  species. 
  If 
  these 
  colonies 
  are 
  

   able 
  to 
  hold 
  their 
  own 
  in 
  the 
  struggle 
  for 
  possession, 
  they 
  will 
  

   multiply 
  in 
  the 
  new 
  conditions, 
  and 
  'the 
  range 
  of 
  the 
  species 
  be- 
  

   comes 
  widened. 
  If 
  the 
  surroundings 
  are 
  different, 
  new 
  species 
  or 
  

   subspecies 
  will 
  be 
  formed 
  with 
  time; 
  and 
  these 
  new 
  forms 
  may 
  

   again 
  invade 
  the 
  territory 
  of 
  the 
  parent 
  species. 
  Again, 
  colony 
  

   after 
  colony 
  of 
  species 
  after 
  species 
  may 
  be 
  destroyed 
  by 
  other 
  

   species 
  or 
  by 
  uncongenial 
  surroundings. 
  

  

  The 
  ultimate 
  result 
  of 
  centuries 
  on 
  centuries 
  of 
  the 
  restlessness 
  of 
  

   individuals 
  is 
  seen 
  in 
  the 
  facts 
  of 
  geographical 
  distribution. 
  Only 
  

   in 
  the 
  most 
  general 
  way 
  can 
  the 
  history 
  of 
  any 
  species 
  be 
  traced; 
  

  

  