﻿RIVERS 
  AS 
  FACTORS 
  IN 
  ANIMAL 
  DISTRIBUTION. 
  189 
  

  

  Most 
  reptiles 
  can 
  SAvim. 
  Dr. 
  Leigh 
  ton 
  has 
  recently 
  made 
  

   some 
  remarks 
  on 
  British 
  species. 
  " 
  It 
  has 
  long 
  been 
  a 
  matter 
  of 
  

   dispute 
  and 
  doubt 
  whether 
  the 
  British 
  Adder 
  (Vipera 
  berus) 
  ever 
  

   took 
  to 
  the 
  water 
  as 
  a 
  matter 
  of 
  ordinary 
  habit. 
  Most 
  ophiologists 
  

   denied 
  this, 
  or 
  at 
  any 
  rate 
  had 
  not 
  observed 
  it. 
  Years 
  of 
  

   observation 
  in 
  English 
  counties 
  had 
  failed 
  to 
  bring 
  forward 
  a 
  

   single 
  case, 
  but 
  the 
  result 
  of 
  some 
  correspondence 
  indicated 
  that 
  

   in 
  Scotland 
  the 
  habit 
  was 
  not 
  unusual. 
  Investigations 
  and 
  

   experiments 
  with 
  Adders 
  in 
  the 
  Scottish 
  Highlands 
  proved 
  that 
  

   in 
  that 
  district 
  Adders 
  were 
  in 
  the 
  habit 
  of 
  swimming 
  the 
  streams 
  

   and 
  rivers, 
  a 
  habit 
  which 
  has 
  become 
  incorporated 
  in 
  some 
  of 
  

   the 
  folklore 
  of 
  the 
  Highlands."* 
  This 
  again 
  indicates 
  that 
  we 
  

   have 
  a 
  paucity 
  of 
  observations 
  relating 
  to 
  the 
  swimming 
  of 
  many 
  

   reptiles. 
  

  

  Even 
  insects 
  of 
  non-aquatic 
  habits 
  swim. 
  Mr. 
  Jas. 
  Hudson, 
  

   in 
  a 
  paper 
  recently 
  read 
  before 
  the 
  Eoyal 
  Horticultural 
  Society 
  

   on 
  the 
  cultivation 
  of 
  Water 
  Lilies, 
  states 
  that 
  Aphides 
  are 
  

   troublesome 
  to 
  Nymphaeas, 
  and 
  that 
  these 
  insects 
  appear 
  to 
  

   have 
  accommodated 
  themselves 
  to 
  the 
  situation, 
  and 
  acquired 
  

   the 
  art 
  of 
  swimming. 
  

  

  We 
  have 
  only 
  given 
  examples 
  of 
  an 
  animal 
  practice, 
  which 
  is 
  

   probably 
  of 
  almost 
  universal 
  application 
  when 
  necessity 
  occurs, 
  

   but 
  which 
  at 
  the 
  same 
  time 
  is 
  one 
  little 
  recorded 
  as 
  regards 
  non- 
  

   aquatic 
  species. 
  In 
  one 
  sense 
  it 
  exhibits 
  the 
  inheritance 
  of 
  an 
  

   acquired 
  habit. 
  All 
  mammals 
  can 
  swim, 
  but 
  few 
  persistently 
  

   exhibit 
  the 
  acquirement. 
  Pursued 
  by 
  enemies, 
  and 
  accidentally 
  

   immersed 
  or 
  borne 
  away 
  by 
  floods, 
  must 
  have 
  been 
  the 
  experi- 
  

   ence 
  of 
  many 
  animals, 
  some 
  of 
  which 
  succumbed, 
  while 
  others 
  

   survived 
  by 
  natatorial 
  capacity. 
  Thus 
  in 
  time, 
  though 
  rivers 
  

   still 
  remained 
  mighty, 
  they 
  were 
  no 
  longer 
  complete 
  barriers 
  to 
  

   the 
  dispersal 
  of 
  land 
  animals, 
  and 
  therefore, 
  though 
  the 
  aggregate 
  

   of 
  a 
  species 
  may 
  be 
  focused 
  on 
  one 
  side 
  of 
  the 
  stream, 
  its 
  migratory 
  

   representatives 
  may 
  be 
  distributed 
  over 
  a 
  considerable 
  area 
  from 
  

   the 
  opposite 
  bank. 
  

  

  * 
  British 
  Association 
  Kept., 
  Sect. 
  D. 
  Southport, 
  1903. 
  

  

  