﻿312 
  AiSTNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1927 
  

  

  in 
  the 
  past, 
  have 
  been 
  effective 
  barriers 
  which 
  have 
  prevented 
  the 
  

   intermingling 
  of 
  groups 
  of 
  peoples. 
  These 
  groups 
  may 
  have 
  origi- 
  

   nated 
  from 
  the 
  same 
  stock, 
  but, 
  lacking 
  contacts 
  one 
  with 
  the 
  other, 
  

   they 
  have 
  developed 
  differently 
  and 
  finally 
  have 
  come 
  to 
  exhibit 
  

   conspicuous 
  differences. 
  

  

  Most 
  of 
  the 
  organisms 
  whose 
  life 
  records 
  have 
  been 
  preserved 
  in 
  the 
  

   rock 
  strata 
  were 
  creatures 
  whose 
  existence 
  was 
  spent 
  in 
  an 
  aquatic 
  

   environment, 
  because 
  the 
  most 
  of 
  our 
  fossil-bearing 
  rocks 
  are 
  con- 
  

   solidated 
  sediments 
  which 
  were 
  laid 
  down 
  in 
  water. 
  Furthermore, 
  

   by 
  far 
  the 
  most 
  of 
  these 
  aquatic 
  sediments, 
  with 
  their 
  inclosed 
  re- 
  

   mains 
  of 
  once 
  living 
  animals, 
  are 
  marine 
  in 
  their 
  origin. 
  Likewise 
  

   most, 
  if 
  not 
  all, 
  of 
  our 
  marine 
  paleontologic 
  record 
  is 
  of 
  compara- 
  

   tively 
  shallow-water 
  organisms. 
  The 
  pathways 
  along 
  which 
  means 
  

   of 
  communication 
  have 
  been 
  established 
  for 
  such 
  shallow-water 
  

   marine 
  life 
  are 
  wholly 
  different 
  from 
  the 
  lines 
  of 
  communication 
  

   suitable 
  for 
  land-living 
  creatures 
  such 
  as 
  men, 
  but 
  the 
  relation 
  of 
  

   lines 
  of 
  communication 
  to 
  the 
  faunal 
  or 
  social 
  evolution 
  of 
  these 
  

   creatures 
  themselves 
  is 
  the 
  same. 
  Among 
  the 
  creatures 
  of 
  the 
  past, 
  

   at 
  times 
  when 
  environmental 
  conditions 
  approached 
  uniformity 
  

   throughout 
  large 
  portions 
  of 
  the 
  earth 
  and 
  when 
  pathways 
  of 
  com- 
  

   munication 
  were 
  relatively 
  open 
  and 
  continuous, 
  a 
  tendency 
  toward 
  

   the 
  development 
  of 
  widespread, 
  more 
  or 
  less 
  uniform 
  faunal 
  assem- 
  

   blages 
  existed, 
  although 
  at 
  no 
  time 
  did 
  such 
  conditions 
  become 
  suf- 
  

   ficiently 
  perfected 
  or 
  continue 
  unbrokenly 
  for 
  long 
  enough 
  time 
  to 
  

   permit 
  the 
  establishment 
  of 
  a 
  truly 
  world-wide, 
  cosmopolitan 
  fauna. 
  

   The 
  Silurian 
  faunas 
  of 
  North 
  America 
  and 
  northern 
  Europe 
  possess 
  

   many 
  common 
  characteristics, 
  showing 
  beyond 
  question 
  that 
  some 
  

   pathway 
  of 
  shallow 
  marine 
  water 
  communication 
  existed 
  during 
  that 
  

   period 
  which 
  permitted 
  the 
  creatures 
  inhabiting 
  these 
  widely 
  sepa- 
  

   rated 
  seas 
  to 
  intermigrate 
  from 
  one 
  region 
  to 
  the 
  other. 
  During 
  a 
  

   portion 
  of 
  Mississippian 
  time 
  the 
  marine 
  faunas 
  of 
  central 
  North 
  

   America 
  and 
  those 
  of 
  northwestern 
  Europe 
  possess 
  much 
  in 
  common, 
  

   to 
  account 
  for 
  which 
  some 
  pathway 
  of 
  intercommunication 
  must 
  be 
  

   assumed 
  to 
  have 
  been 
  in 
  existence. 
  Many 
  other 
  examples 
  of 
  close 
  

   similarity 
  between 
  extinct 
  contemporaneous 
  marine 
  faunas, 
  in 
  widely 
  

   separated 
  portions 
  of 
  the 
  earth, 
  are 
  well 
  known, 
  and 
  the 
  existence 
  of 
  

   such 
  similarities 
  is 
  uniformly 
  accepted 
  by 
  paleontologists 
  as 
  evidence 
  

   for 
  the 
  existence 
  of 
  some 
  pathway 
  of 
  communication 
  between 
  these 
  

   distant 
  localities. 
  

  

  Likewise 
  the 
  animals 
  and 
  plants 
  which 
  have 
  lived 
  on 
  the 
  land 
  have 
  

   shown 
  relationships 
  over 
  widely 
  separated 
  geographic 
  areas 
  at 
  certain 
  

   periods. 
  During 
  upper 
  Miocene 
  time 
  the 
  land 
  mammalian 
  faunas 
  of 
  

   North 
  America 
  and 
  Europe 
  are 
  known 
  to 
  have 
  included 
  so 
  many 
  

   closely 
  related 
  forms 
  that 
  the 
  only 
  possible 
  explanation 
  of 
  the 
  situa- 
  

  

  