﻿iPALEONTOLOGY 
  — 
  WELLER 
  313 
  

  

  tion 
  is 
  the 
  assumption 
  of 
  the 
  existence 
  of 
  a 
  so-called 
  land 
  bridge 
  over 
  

   which 
  these 
  creatures 
  could 
  pass. 
  As 
  the 
  existence 
  of 
  such 
  a 
  line 
  of 
  

   communication 
  could 
  only 
  have 
  been 
  in 
  the 
  far 
  north, 
  Ave 
  are 
  forced 
  

   to 
  the 
  conclusion 
  that 
  the 
  climatic 
  conditions 
  of 
  the 
  period 
  were 
  much 
  

   different 
  from 
  those 
  which 
  now 
  obtain. 
  At 
  this 
  time 
  the 
  primitive 
  

   horses 
  seem 
  to 
  have 
  migrated 
  from 
  America 
  to 
  Europe, 
  while 
  the 
  

   primitive 
  elephant-like 
  mastodons 
  found 
  their 
  way 
  from 
  Europe 
  into 
  

   America. 
  Many 
  other 
  manmialian 
  types, 
  including 
  members 
  of 
  the 
  

   rhinoceros 
  and 
  tapir 
  families, 
  are 
  also 
  common 
  to 
  the 
  two 
  continents 
  

   at 
  this 
  time. 
  

  

  During 
  Carboniferous 
  time 
  the 
  plant 
  life 
  of 
  North 
  AnTerica 
  and 
  

   Europe 
  possessed 
  many 
  common 
  characteristics. 
  Many 
  of 
  the 
  same 
  

   genera 
  and 
  the 
  same 
  species 
  are 
  recognized 
  in 
  the 
  fossil 
  floras 
  of 
  these 
  

   widely 
  separated 
  regions, 
  and 
  they 
  could 
  only 
  have 
  had 
  such 
  geo- 
  

   graphic 
  distribution 
  with 
  lines 
  of 
  land 
  communication 
  somewhere 
  

   between 
  the 
  two 
  continents. 
  

  

  Some 
  periods 
  in 
  geologic 
  history 
  show 
  notable 
  provincialism 
  in 
  the 
  

   development 
  of 
  life 
  on 
  the 
  earth. 
  The 
  northern 
  and 
  southern 
  Ameri- 
  

   can 
  continents 
  were 
  essentially 
  isolated, 
  so 
  far 
  as 
  their 
  land 
  life 
  was 
  

   concerned 
  during 
  much 
  of 
  Tertiary 
  time, 
  known 
  as 
  the 
  age 
  of 
  mam- 
  

   mals, 
  so 
  that 
  in 
  South 
  America, 
  especially, 
  a 
  notable 
  provincial 
  land 
  

   fauna 
  developed. 
  About 
  the 
  beginning 
  of 
  Tertiary 
  time 
  there 
  seems 
  

   to 
  have 
  been 
  a 
  limited 
  connection 
  between 
  the 
  two 
  continents, 
  follow- 
  

   ing 
  which 
  isolation 
  of 
  the 
  southern 
  continent 
  is 
  believed 
  to 
  have 
  been 
  

   complete 
  until 
  Miocene 
  time, 
  when 
  a 
  few 
  of 
  the 
  peculiar 
  southern 
  

   types 
  began 
  to 
  filter 
  through 
  into 
  the 
  more 
  northern 
  continent. 
  

  

  The 
  most 
  isolated 
  continental 
  area 
  of 
  the 
  present 
  time 
  is 
  Australia, 
  

   and 
  because 
  it 
  has 
  lacked 
  any 
  land 
  connections 
  with 
  other 
  continents 
  

   for 
  so 
  long 
  a 
  time, 
  it 
  has 
  come 
  to 
  be 
  inhabited 
  by 
  a 
  most 
  peculiar, 
  

   provincial 
  land 
  fauna. 
  If 
  means 
  of 
  communication 
  between 
  Aus- 
  

   tralia 
  and 
  its 
  neighboring 
  continents 
  had 
  existed, 
  its 
  marsupial 
  fauna 
  

   undoubtedly 
  would 
  have 
  been 
  much 
  modified 
  and 
  perhaps 
  completely 
  

   exterminated 
  by 
  the 
  more 
  advanced 
  mammalian 
  types. 
  

  

  During 
  Devonian 
  time 
  a 
  number 
  of 
  distinct 
  marine 
  faunal 
  prov- 
  

   inces 
  are 
  known 
  to 
  have 
  existed 
  within 
  the 
  limits 
  of 
  the 
  North 
  

   American 
  Continent, 
  each 
  one 
  inhabited 
  by 
  an 
  assemblage 
  of 
  creatures 
  

   distinctly 
  different 
  from 
  those 
  in 
  another 
  province 
  not 
  greatly 
  re- 
  

   moved 
  geographically, 
  but 
  separated, 
  nevertheless, 
  by 
  a 
  barrier 
  which 
  

   was 
  insurmountable 
  to 
  the 
  creatures 
  whose 
  fossil 
  remains 
  we 
  now 
  

   dig 
  out 
  of 
  the 
  rocks. 
  With 
  changes 
  in 
  the 
  course 
  and 
  positions 
  of 
  

   the 
  strandlines 
  of 
  this 
  period, 
  notable 
  changes 
  in 
  the 
  distribution 
  of 
  

   the 
  faunas 
  themselves 
  came 
  about. 
  The 
  story 
  of 
  the 
  faunal 
  changes 
  

   and 
  migration 
  of 
  this 
  ancient 
  Devonian 
  life, 
  read 
  from 
  the 
  paleon- 
  

   tological 
  records 
  in 
  the 
  sedimentary 
  rocks, 
  is 
  as 
  fascinating 
  as 
  the 
  

  

  