﻿DISTEIBUTION 
  OF 
  FISHES 
  JORDAN 
  379 
  

  

  nebulosus) 
  , 
  an 
  immigrant 
  perhaps 
  from 
  South 
  America, 
  and 
  which 
  

   has 
  now 
  adjusted 
  itself 
  to 
  its 
  surroundings 
  in 
  each 
  of 
  our 
  myriad 
  

   catfish 
  breeding 
  streams. 
  

  

  The 
  word 
  " 
  species," 
  then, 
  is 
  simply 
  a 
  term 
  of 
  convenience, 
  in- 
  

   cluding 
  such 
  members 
  of 
  a 
  group 
  similar 
  to 
  each 
  other 
  as 
  are 
  tangibly 
  

   different 
  from 
  others, 
  which 
  have 
  endured 
  in 
  the 
  gauntlet 
  of 
  life 
  

   and 
  are 
  not 
  known 
  to 
  be 
  still 
  connected 
  with 
  these 
  by 
  intermediate 
  

   forms. 
  Such 
  connecting 
  links 
  we 
  may 
  suppose 
  to 
  have 
  existed 
  in 
  

   all 
  cases. 
  We 
  can 
  only 
  be 
  sure 
  that 
  they 
  do 
  not 
  now 
  exist 
  in 
  our 
  

   collections, 
  so 
  far 
  as 
  these 
  have 
  been 
  carefully 
  studied. 
  

  

  When 
  two 
  or 
  more 
  species 
  of 
  any 
  genus 
  now 
  inhabit 
  the 
  same 
  

   waters, 
  they 
  are 
  usually 
  species 
  whose 
  differentiation 
  is 
  of 
  long 
  

   standing 
  — 
  species, 
  therefore, 
  which 
  can 
  be 
  readily 
  distinguished 
  

   from 
  one 
  another. 
  When, 
  on 
  the 
  other 
  hand, 
  we 
  have 
  "repre- 
  

   sentative 
  species" 
  (better 
  called 
  geminate 
  or 
  twin 
  species) 
  — 
  closely 
  

   related 
  forms, 
  neither 
  of 
  which 
  is 
  found 
  within 
  the 
  geograpical 
  

   range 
  of 
  the 
  other 
  — 
  we 
  can 
  with 
  some 
  confidence 
  look 
  for 
  interme- 
  

   diate 
  forms 
  where 
  the 
  territory 
  occupied 
  by 
  the 
  one 
  bounds 
  that 
  

   inhabited 
  by 
  the 
  other. 
  In 
  very 
  many 
  such 
  cases 
  intermediate 
  forms 
  

   have 
  been 
  found 
  ; 
  and 
  such 
  forms 
  are 
  considered 
  as 
  subspecies 
  of 
  one 
  

   species, 
  the 
  one 
  being 
  regarded 
  as 
  the 
  parent 
  stock, 
  the 
  other 
  as 
  an 
  

   offshoot 
  due 
  to 
  separation 
  under 
  influences 
  of 
  different 
  environ- 
  

   ment. 
  Then, 
  besides 
  these 
  " 
  species 
  " 
  and 
  " 
  subspecies," 
  groups 
  more 
  

   or 
  less 
  readily 
  recognizable, 
  there 
  are 
  varieties 
  and 
  variations 
  of 
  

   every 
  grade, 
  often 
  too 
  ill 
  defined 
  to 
  receive 
  any 
  sort 
  of 
  name, 
  but 
  still 
  

   not 
  without 
  significance 
  to 
  the 
  student 
  of 
  the 
  origin 
  of 
  species. 
  

   Comparing 
  a 
  dozen 
  fresh 
  specimens 
  of 
  almost 
  any 
  kind 
  of 
  fish 
  from 
  

   any 
  body 
  of 
  water 
  with 
  an 
  equal 
  number 
  from 
  somewhere 
  else, 
  one 
  

   will 
  rarely 
  fail 
  to 
  find 
  some 
  sort 
  of 
  differences 
  — 
  in 
  size, 
  in 
  form, 
  in 
  

   color. 
  These 
  differences 
  are 
  obviously 
  the 
  reflex 
  of 
  differences 
  in 
  

   the 
  environment, 
  due 
  to 
  segregation 
  and 
  to 
  alterations 
  in 
  the 
  stress 
  

   of 
  selection. 
  The 
  collector 
  of 
  fishes 
  should 
  recognize 
  ontogenetic 
  

   (not 
  hereditary) 
  alterations. 
  It 
  is 
  usually 
  not 
  difficult 
  to 
  refer 
  the 
  

   effects 
  to 
  causes. 
  Thus, 
  fishes 
  from 
  grassy 
  bottoms 
  are 
  darker 
  than 
  

   those 
  taken 
  from 
  over 
  sand, 
  and 
  those 
  from 
  a 
  bottom 
  of 
  muck 
  are 
  

   darker 
  still, 
  the 
  shade 
  of 
  color 
  being, 
  in 
  some 
  way 
  not 
  well 
  under- 
  

   stood, 
  dependent 
  on 
  the 
  color 
  of 
  the 
  surroundings. 
  Fishes 
  in 
  large 
  

   bodies 
  of 
  water 
  reach 
  a 
  larger 
  size 
  than 
  the 
  same 
  species 
  in 
  smaller 
  

   streams 
  or 
  ponds. 
  Fishes 
  from 
  foul 
  or 
  sediment-laden 
  waters 
  are 
  

   paler 
  in 
  color 
  and 
  slenderer 
  in 
  form 
  than 
  those 
  from 
  waters 
  which 
  

   are 
  clear 
  and 
  pure. 
  Again, 
  it 
  is 
  often 
  true 
  that 
  specimens 
  from 
  

   northern 
  waters 
  are 
  less 
  slender 
  in 
  body 
  than 
  those 
  from 
  farther 
  

   south. 
  Other 
  things 
  being 
  equal, 
  the 
  more 
  remote 
  the 
  localities 
  

   from 
  each 
  other, 
  the 
  greater 
  are 
  these 
  differences. 
  

  

  