﻿RIVERS 
  AS 
  FACTORS 
  IN 
  ANIMAL 
  DISTRIBUTION. 
  123 
  

  

  animal 
  distribution. 
  The 
  interesting 
  history 
  of 
  European 
  

   rivers 
  still 
  remains 
  to 
  be 
  written. 
  They 
  did 
  not 
  always 
  flow 
  

   along 
  their 
  present 
  courses, 
  and 
  the 
  Bhone 
  appears 
  to 
  have 
  been 
  

   a 
  great 
  traveller. 
  At 
  least 
  there 
  seems 
  reason 
  to 
  believe 
  that 
  

   the 
  upper 
  waters 
  of 
  the 
  Valais 
  fell 
  at 
  first 
  into 
  the 
  Danube, 
  and 
  

   so 
  into 
  the 
  Black 
  Sea 
  ; 
  subsequently 
  joined 
  the 
  Rhine 
  and 
  the 
  

   Thames, 
  and 
  so 
  ran 
  far 
  north 
  over 
  the 
  plains 
  which 
  once 
  con- 
  

   nected 
  the 
  mountains 
  of 
  Scotland 
  and 
  of 
  Norway 
  to 
  the 
  Arctic 
  

   Ocean, 
  and 
  have 
  only 
  comparatively 
  of 
  late 
  years 
  adopted 
  their 
  

   present 
  course 
  into 
  the 
  Mediterranean.* 
  

  

  As 
  a 
  general 
  rule 
  even 
  narrow 
  seas 
  are 
  an 
  effectual 
  obstacle 
  

   to 
  migration 
  ; 
  "in 
  many 
  cases 
  wide 
  rivers 
  are 
  equally 
  effective 
  

   barriers." 
  f 
  In 
  Queensland, 
  according 
  to 
  C. 
  Lumholtz 
  : 
  "About 
  

   250 
  miles 
  from 
  the 
  coast 
  we 
  passed 
  the 
  part 
  of 
  the 
  Great 
  Dividing 
  

   Range 
  which 
  here 
  forms 
  the 
  watershed 
  between 
  Eastern 
  and 
  

   Western 
  Queensland. 
  In 
  this 
  part 
  the 
  watershed 
  consists 
  of 
  a 
  

   low 
  range. 
  Nevertheless, 
  no 
  one 
  can 
  fail 
  to 
  observe 
  the 
  great 
  

   difference 
  in 
  animal 
  life 
  on 
  the 
  two 
  sides, 
  as 
  well 
  as 
  the 
  inter- 
  

   great 
  upheavals 
  and 
  changes 
  in 
  the 
  land 
  took 
  place 
  (changes 
  that 
  were 
  

   probably 
  most 
  acute 
  in 
  the 
  immense 
  water 
  systems 
  of 
  the 
  Zambesi 
  and 
  

   Xgami 
  countries, 
  before 
  the 
  Zambesi 
  was 
  torn 
  from 
  its 
  ancient 
  southerly 
  

   course 
  and 
  diverted 
  to 
  its 
  present 
  channel), 
  Bechuanaland 
  and 
  the 
  Kalahari 
  

   must 
  have 
  been 
  exceedingly 
  well 
  watered. 
  Many 
  an 
  old 
  rivershed, 
  long 
  

   since 
  dry 
  and 
  useless, 
  testifies 
  to 
  the 
  fact 
  " 
  (ibid. 
  p. 
  115). 
  During 
  the 
  great 
  

   drought 
  of 
  1S62, 
  "the 
  mighty 
  Orange 
  River 
  could 
  be 
  stepped 
  across 
  by 
  a 
  

   child, 
  and 
  in 
  its 
  upper 
  part 
  at 
  least 
  ran 
  dry, 
  exposing 
  in 
  its 
  bed 
  near 
  Hope 
  

   Town 
  the 
  remains 
  of 
  a 
  waggon 
  which 
  had 
  been 
  lost 
  in 
  a 
  sudden 
  flood 
  while 
  

   crossing 
  the 
  river 
  some 
  thirty 
  years 
  before 
  " 
  (cf. 
  ' 
  Hydrology 
  of 
  South 
  Africa,' 
  

   by 
  J. 
  Crumbie 
  Brown, 
  p. 
  112). 
  Mr. 
  Sharpe, 
  in 
  an 
  article 
  on 
  British 
  Central 
  

   Africa, 
  writes 
  : 
  — 
  " 
  With 
  regard 
  to 
  the 
  drying 
  up 
  or 
  otherwise 
  of 
  African 
  lakes 
  

   and 
  streams, 
  I 
  am 
  inclined 
  to 
  doubt 
  the 
  theory 
  that 
  there 
  is 
  a 
  desiccation 
  of 
  a 
  

   permanent 
  kind 
  going 
  on 
  — 
  that 
  is, 
  I 
  mean, 
  a 
  drying 
  up 
  due 
  to 
  a 
  permanently 
  

   decreasing 
  rainfall. 
  It 
  is, 
  I 
  think, 
  more 
  reasonable 
  to 
  suppose 
  that, 
  owing 
  

   to 
  some 
  cause 
  which 
  is 
  not 
  as 
  yet 
  apparent, 
  there 
  are 
  regular 
  cycles 
  in 
  the 
  

   lake 
  regions 
  of 
  Africa, 
  due 
  to 
  increasing 
  and 
  decreasing 
  rainfalls 
  " 
  (' 
  Geo- 
  

   graphical 
  Journal,' 
  vol. 
  vii. 
  p. 
  367). 
  That 
  a 
  full 
  average 
  rainfall 
  is 
  essential 
  

   is 
  proved 
  by 
  the 
  statement 
  of 
  Anderson 
  in 
  Damara 
  Land, 
  that 
  so 
  great 
  is 
  the 
  

   rate 
  of 
  absorption 
  and 
  evaporation 
  at 
  this 
  season 
  (December 
  and 
  January), 
  

   " 
  that 
  pools 
  of 
  from 
  forty 
  to 
  fifty 
  feet 
  long, 
  and 
  several 
  feet 
  deep, 
  would 
  dry 
  

   up 
  in 
  the 
  course 
  of 
  a 
  week 
  " 
  (' 
  The 
  Okavango 
  Paver,' 
  p. 
  146). 
  

  

  * 
  Cf. 
  Lord 
  Avebury, 
  ' 
  The 
  Pleasures 
  of 
  Life,' 
  pt. 
  i. 
  p. 
  127-8. 
  

  

  f 
  Dixon, 
  ' 
  The 
  Migration 
  of 
  British 
  Birds,' 
  p. 
  102. 
  

  

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