﻿PALAEONTOLOGY 
  129 
  

  

  Epigene 
  or 
  superficial 
  agencies 
  

   Air 
  

  

  „_ 
  , 
  ( 
  terrestrial 
  

   Water 
  \ 
  

  

  { 
  oceanic 
  

   Erosion 
  and 
  sedimentation 
  

   Animal 
  and 
  plant 
  life. 
  

  

  PALAEONTOLOGY 
  

  

  In 
  studying 
  an 
  extensive 
  series 
  of 
  geologic 
  formations 
  from 
  

   bottom 
  to 
  top, 
  we 
  find 
  that 
  through 
  geologic 
  time 
  there 
  has 
  been 
  

   a 
  progressive 
  advance 
  in 
  the 
  development 
  of 
  animal 
  and 
  plant 
  

   life 
  as 
  well 
  as 
  a 
  change 
  of 
  genera 
  and 
  species. 
  Forms 
  that 
  are 
  

   abundant 
  at 
  one 
  horizon 
  seem 
  to 
  have 
  ultimately 
  given 
  up 
  the 
  

   battle 
  for 
  existence 
  and 
  disappeared, 
  their 
  place 
  in 
  nature 
  being 
  

   filled 
  by 
  others. 
  So 
  by 
  careful 
  comparison 
  of 
  the 
  animal 
  and 
  

   vegetable 
  remains 
  found 
  in 
  the 
  different 
  systems 
  and 
  groups 
  and 
  

   in 
  the 
  minor 
  sub-divisions 
  of 
  the 
  groups, 
  we 
  come 
  to 
  regard 
  the 
  

   fossils 
  as 
  labels 
  by 
  which 
  we 
  may 
  know 
  the 
  age 
  of 
  strata. 
  While 
  

   there 
  are 
  some 
  persistent 
  types 
  which 
  pass 
  from 
  one 
  system 
  to 
  

   another 
  without 
  material 
  change, 
  we 
  find 
  that 
  the 
  life 
  character- 
  

   istics 
  of 
  each 
  group 
  are 
  essentially 
  distinct. 
  It 
  is 
  therefore 
  im- 
  

   portant 
  for 
  the 
  field 
  geologist 
  who 
  is 
  studying 
  the 
  formations 
  

   above 
  the 
  Archaean 
  to 
  be 
  familiar 
  with 
  their 
  fossils 
  in 
  order 
  to 
  

   determine 
  the 
  horizons 
  accurately. 
  

  

  In 
  the 
  older 
  formations, 
  plants 
  were 
  few 
  and 
  elementary 
  and, 
  

   containing 
  but 
  little 
  mineral 
  matter, 
  have 
  not 
  been 
  well 
  preserved 
  

   so 
  that 
  we 
  depend 
  more 
  on 
  fossil 
  animals 
  than 
  plants 
  for 
  the 
  

   identification 
  of 
  the 
  Palaeozoic 
  strata. 
  From 
  the 
  Mesozoic 
  on, 
  

   impressions 
  of 
  land 
  plants 
  are 
  more 
  abundant 
  and 
  become 
  of 
  

   much 
  value 
  in 
  palaeontology. 
  

  

  As 
  shown 
  by 
  the 
  fossil 
  remains 
  discovered 
  in 
  rocks 
  of 
  different 
  

   ages, 
  the 
  development 
  of 
  animal 
  life 
  has 
  been 
  a 
  gradual 
  one, 
  but 
  

   we 
  are 
  not 
  yet 
  acquainted 
  with 
  any 
  formation 
  which 
  contains 
  the 
  

   earliest 
  forms 
  of 
  life. 
  We 
  begin 
  our 
  study, 
  as 
  it 
  were, 
  at 
  a 
  some- 
  

   what 
  late 
  period 
  of 
  life 
  development, 
  the 
  Cambrian, 
  for 
  the 
  fossils 
  

   of 
  the 
  pre-Cambrian 
  rocks 
  are 
  not 
  yet 
  well 
  known. 
  Somewhere 
  

  

  