﻿FOSSIL 
  MARINE 
  FAUNAS 
  KIRK 
  305 
  

  

  for 
  the 
  higher 
  types 
  of 
  invertebrates 
  and 
  even 
  some 
  of 
  the 
  verte- 
  

   brates. 
  The 
  abundance 
  of 
  the 
  phytoplankton, 
  in 
  turn, 
  is 
  dependent 
  

   on 
  the 
  presence 
  of 
  a 
  long 
  list 
  of 
  chemical 
  elements 
  in 
  the 
  sea 
  water. 
  

   It 
  seems 
  to 
  have 
  been 
  shown 
  that 
  of 
  these 
  the 
  presence 
  or 
  absence 
  of 
  

   nitrogen 
  is 
  the 
  most 
  critical 
  factor, 
  the 
  other 
  elements 
  being 
  more 
  

   stable 
  and 
  of 
  more 
  universal 
  distribution. 
  Analyses 
  have 
  shown 
  a 
  

   low 
  nitrogen 
  content 
  in 
  many 
  of 
  the 
  warmer 
  sea 
  waters, 
  probably 
  due. 
  

   to 
  the 
  greater 
  activity 
  of 
  denitrifying 
  bacteria. 
  Here 
  we 
  seem 
  to 
  

   have 
  the 
  explanation 
  of 
  the 
  relative 
  scarcity 
  of 
  life 
  in 
  some 
  of 
  the 
  

   .warm-water 
  regions. 
  Apart 
  from 
  the 
  factor 
  of 
  food, 
  marine 
  life 
  

   thrives 
  in 
  varying 
  degrees 
  under 
  widely 
  varying 
  conditions 
  of 
  tem- 
  

   perature, 
  salinity, 
  light, 
  pressure, 
  and 
  mechanical 
  stress. 
  Extreme 
  

   variation 
  from 
  the 
  average 
  of 
  any 
  of 
  these 
  factors 
  is 
  harmful 
  but 
  

   seldom 
  under 
  natural 
  conditions 
  prohibitive. 
  It 
  is 
  a 
  constant 
  source 
  

   of 
  surprise 
  under 
  what 
  uncongenial 
  conditions 
  animals 
  of 
  some 
  sort 
  

   will 
  thrive 
  and 
  multiply. 
  Too 
  little 
  account 
  is 
  taken 
  of 
  the 
  great 
  

   adaptability 
  of 
  organisms 
  to 
  changing 
  environment 
  when 
  plenty 
  of 
  

   time 
  is 
  allowed. 
  

  

  It 
  is 
  of 
  course 
  not 
  possible 
  to 
  solve 
  the 
  problem 
  of 
  barren 
  sedi- 
  

   mentary 
  rocks 
  of 
  the 
  past 
  by 
  any 
  general 
  formula. 
  Apart 
  from 
  

   food, 
  the 
  presence 
  of 
  oxygen, 
  reasonably 
  clean 
  water, 
  and 
  the 
  ab- 
  

   sence 
  of 
  positively 
  poisonous 
  substances 
  are 
  perhaps 
  the 
  most 
  im- 
  

   portant 
  requirements 
  of 
  marine 
  animals. 
  In 
  semilandlocked 
  basins, 
  

   or 
  those 
  having 
  a 
  threshold 
  barrier 
  making 
  for 
  poor 
  water 
  circulation, 
  

   lack 
  of 
  oxygen 
  may 
  obtain, 
  or 
  the 
  generation 
  of 
  sulphureted 
  hydro- 
  

   gen 
  from 
  the 
  decay 
  of 
  organisms 
  may 
  render 
  life 
  on 
  the 
  bottom 
  im- 
  

   possible 
  except 
  for 
  bacteria. 
  Such 
  conditions 
  are 
  found 
  in 
  the 
  Black 
  

   and 
  Caspian 
  Seas 
  as 
  well 
  as 
  in 
  some 
  of 
  the 
  Norwegian 
  fiords. 
  As 
  a 
  

   matter 
  of 
  fact 
  perhaps 
  the 
  greater 
  part 
  of 
  the 
  barren 
  shales 
  may 
  

   most 
  readily 
  be 
  explained 
  as 
  the 
  result 
  of 
  foul 
  water. 
  Muds 
  within 
  

   the 
  zone 
  of 
  wave 
  action, 
  that 
  is, 
  approximately 
  100 
  fathoms, 
  are 
  

   kept 
  more 
  or 
  less 
  stirred 
  up 
  and 
  in 
  a 
  partial 
  state 
  of 
  suspension. 
  This 
  

   is 
  a 
  condition 
  under 
  which 
  few 
  animals 
  can 
  survive. 
  At 
  any 
  rate 
  I 
  

   think 
  we 
  may 
  safely 
  assume 
  that 
  cold 
  climate 
  has 
  a 
  very 
  remote 
  

   chance 
  of 
  having 
  a 
  great 
  deal 
  to 
  do 
  with 
  the 
  lack 
  or 
  rarity 
  of 
  marine 
  

   life, 
  and 
  even 
  water 
  temperature 
  is 
  not 
  a 
  critical 
  factor. 
  

  

  There 
  is 
  at 
  present 
  one 
  outstanding 
  difference 
  between 
  cold 
  and 
  

   warm 
  water 
  faunas. 
  This 
  is 
  that 
  despite 
  the 
  great 
  abundance 
  of 
  in- 
  

   dividuals 
  in 
  cold-water 
  faunas 
  the 
  number 
  of 
  species 
  is 
  relatively 
  

   less. 
  Of 
  how 
  little 
  use 
  this 
  criterion 
  is 
  in 
  evaluating 
  a 
  faunule 
  of 
  

   the 
  past 
  may 
  be 
  judged 
  by 
  considering 
  the 
  fauna 
  of 
  the 
  Antarctic. 
  

   With 
  its 
  275 
  genera 
  it 
  is 
  sufficiently 
  rich 
  to 
  compare 
  favorably 
  with 
  

   any 
  fossil 
  fauna, 
  and 
  even 
  eliminating 
  the 
  soft-bodied 
  types 
  which 
  

   are 
  seldom 
  preserved 
  in 
  the 
  rocks 
  it 
  still 
  does 
  very 
  well. 
  In 
  this 
  

  

  