﻿180 
  THE 
  ZOOLOGIST. 
  

  

  BIOLOGICAL 
  SUGGESTIONS. 
  

   RIVERS 
  AS 
  FACTORS 
  IN 
  ANIMAL 
  DISTRIBUTION. 
  

  

  By 
  W. 
  L. 
  Distant. 
  

  

  (Concluded 
  from 
  p. 
  133.) 
  

  

  Part 
  II. 
  — 
  Distributive 
  Action. 
  

  

  If 
  rivers 
  have 
  proved 
  barriers 
  to 
  the 
  distribution 
  of 
  animals 
  

   in 
  one 
  way, 
  they 
  have 
  also 
  assisted 
  in 
  another 
  manner 
  to 
  the 
  

   dispersal 
  of 
  much 
  animal 
  life. 
  As 
  Heilprin 
  has 
  well 
  remarked 
  : 
  

   " 
  In 
  regions 
  like 
  the 
  tropics, 
  which 
  support 
  a 
  luxuriant 
  vegetable 
  

   growth, 
  and 
  which 
  are 
  subject 
  to 
  periodical 
  overflows, 
  and, 
  con- 
  

   sequently, 
  to 
  the 
  uprooting 
  or 
  outwashing 
  action 
  of 
  the 
  in- 
  

   undating 
  waters, 
  it 
  not 
  infrequently 
  happens 
  that 
  islands 
  or 
  

   ' 
  rafts 
  ' 
  of 
  considerable 
  magnitude, 
  consisting 
  mainly 
  of 
  inter- 
  

   laced 
  or 
  matted 
  vegetation 
  — 
  tree-trunks 
  held 
  together 
  by 
  various 
  

   creepers 
  and 
  climbers, 
  and 
  containing 
  a 
  sufficient 
  quantity 
  of 
  

   vegetable 
  mould 
  and 
  soil 
  bound 
  together 
  in 
  the 
  roots 
  — 
  are 
  floated 
  

   down 
  stream 
  into 
  the 
  open 
  sea, 
  where 
  they 
  are 
  at 
  once 
  placed 
  at 
  

   the 
  mercy 
  of 
  the 
  prevailing 
  oceanic 
  and 
  atmospheric 
  currents."* 
  

   Many 
  such 
  occurrences 
  have 
  been 
  detailed 
  by 
  competent 
  ob- 
  

   servers, 
  and 
  have 
  not 
  been 
  confined 
  to 
  the 
  tropics. 
  t 
  We 
  can 
  

   scarcely 
  over-estimate 
  the 
  amount 
  of 
  animal 
  life 
  contained 
  in 
  

   this 
  mighty 
  flotsam 
  and 
  jetsam. 
  The 
  northern 
  coast 
  of 
  Spitz- 
  

   bergen 
  is 
  covered 
  with 
  immense 
  accumulations 
  of 
  driftwood, 
  

   which 
  has 
  been 
  pronounced 
  by 
  botanists 
  to 
  be 
  nearly 
  all 
  Siberian 
  

   larch, 
  brought 
  by 
  ocean 
  currents 
  from 
  the 
  mouths 
  of 
  the 
  great 
  

  

  :;: 
  ' 
  Geogr. 
  and 
  Geol. 
  Dist. 
  Animals,' 
  p. 
  44. 
  

  

  •| 
  Similar 
  floating 
  islands 
  are 
  found 
  on 
  lakes. 
  Capt. 
  Gilliss 
  states 
  that 
  

   before 
  Lake 
  Taguataga 
  in 
  Chili 
  was 
  drained 
  there 
  were 
  in 
  it 
  islands 
  composed 
  

   of 
  dead 
  plants 
  matted 
  together 
  to 
  a 
  thickness 
  of 
  from 
  four 
  to 
  six 
  feet, 
  and 
  with 
  

   trees 
  of 
  medium 
  size 
  growing 
  upon 
  them. 
  These 
  islands 
  floated 
  before 
  the 
  

   wind, 
  " 
  with 
  their 
  trees 
  and 
  browsing 
  cattle 
  " 
  {cf. 
  Marsh, 
  ' 
  Man 
  and 
  Nature,' 
  

   p. 
  .">.">0, 
  note). 
  

  

  