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  common 
  tape-worm 
  lays 
  hundreds 
  of 
  millions 
  of 
  eggs 
  in 
  a 
  

   very 
  short 
  time, 
  yet 
  it 
  is 
  comparatively 
  rare. 
  The 
  number 
  of 
  

   children 
  born 
  to 
  each 
  pair 
  of 
  human 
  beings 
  during 
  their 
  life- 
  

   time 
  of 
  from 
  fifty 
  to 
  eighty 
  years, 
  can 
  be 
  counted 
  on 
  the 
  

   fingers, 
  yet 
  man 
  is 
  the 
  most 
  abundant 
  of 
  the 
  larger 
  mammals, 
  

   and 
  human 
  population 
  increases 
  quite 
  rapidly 
  under 
  favora- 
  

   ble 
  circumstances. 
  This 
  comparison 
  shows 
  plainly 
  that 
  the 
  

   abundance 
  of 
  a 
  species 
  is 
  determined, 
  mainly, 
  by 
  the 
  exter- 
  

   nal 
  conditions 
  to 
  which 
  it 
  is 
  exposed, 
  and 
  that 
  the 
  num- 
  

   ber 
  of 
  individuals 
  which 
  are 
  born 
  has 
  very 
  little 
  to 
  do 
  with 
  

   it. 
  In 
  the 
  case 
  of 
  the 
  oyster, 
  the 
  adult 
  is 
  well 
  protected 
  

   against 
  enemies 
  by 
  the 
  shell, 
  and 
  as 
  its 
  food 
  is 
  abundant, 
  and 
  

   is 
  brought 
  to 
  it 
  by 
  the 
  water, 
  it 
  is 
  tolerably 
  sure 
  of 
  a 
  long 
  

   life 
  after 
  it 
  has 
  reached 
  its 
  adult 
  form, 
  but 
  the 
  life 
  of 
  the 
  

   young 
  is 
  very 
  precarious 
  ; 
  that 
  of 
  the 
  young 
  American 
  oyster 
  

   peculiarly 
  so, 
  since 
  it 
  is 
  exposed 
  to 
  all 
  kinds 
  of 
  enemies 
  and 
  

   accidents, 
  at 
  a 
  time 
  when 
  it 
  is 
  most 
  helpless. 
  The 
  protection 
  

   of 
  the 
  young 
  European 
  oyster 
  by 
  the 
  parent 
  shell 
  at 
  this 
  time 
  

   would 
  seem 
  to 
  more 
  than 
  balance 
  the 
  greater 
  number 
  of 
  eggs 
  

   laid 
  by 
  the 
  American. 
  

  

  The 
  most 
  critical 
  time 
  in 
  the 
  life 
  of 
  the 
  American 
  oyster 
  is 
  

   undoubtedly 
  the 
  time 
  when 
  the 
  egg 
  is 
  discharged 
  into 
  the 
  

   water 
  to 
  be 
  fertilized, 
  for 
  the 
  chance 
  that 
  each 
  egg 
  which 
  

   floats 
  out 
  into 
  the 
  ocean 
  to 
  shift 
  for 
  itself 
  will 
  immediately 
  

   meet 
  with 
  a 
  male 
  cell 
  is 
  very 
  slight, 
  and 
  it 
  is 
  essential 
  that 
  

   the 
  egg 
  should 
  be 
  fertilized 
  very 
  quickly, 
  for 
  the 
  unfertilized 
  

   egg 
  is 
  destroyed 
  by 
  the 
  sea 
  water 
  in 
  a 
  very 
  short 
  time. 
  The 
  

   next 
  period 
  of 
  great 
  danger 
  is 
  the 
  short 
  time 
  during 
  which 
  

   the 
  embryos 
  swarm 
  to 
  the 
  surface 
  of 
  the 
  water. 
  They 
  are 
  so 
  

   perfectly 
  defenceless, 
  and 
  so 
  crowded 
  together 
  close 
  to 
  the 
  sur- 
  

   face, 
  that 
  a 
  small 
  fish, 
  swimming 
  along 
  with 
  open 
  mouth, 
  

   might 
  easily 
  swallow 
  in 
  a 
  few 
  mouthfulls 
  a 
  number 
  equal 
  to 
  

   the 
  human 
  population 
  of 
  Baltimore. 
  They 
  are 
  also 
  exposed 
  

   to 
  sudden 
  changes 
  of 
  temperature, 
  and 
  as 
  my 
  experiments 
  

   have 
  shown 
  that 
  a 
  sudden 
  fall 
  in 
  temperature 
  is 
  fatal 
  to 
  them 
  

   at 
  this 
  time, 
  the 
  number 
  which 
  are 
  destroyed 
  by 
  cold 
  rains 
  

   and 
  winds 
  must 
  be 
  very 
  great 
  indeed. 
  

  

  As 
  soon 
  as 
  they 
  are 
  safely 
  past 
  this 
  stage, 
  and 
  scatter 
  and 
  

  

  