﻿G. 
  R. 
  Wieland 
  — 
  On 
  Liassic 
  Floras. 
  263 
  

  

  underlying 
  " 
  Midford 
  sands." 
  These 
  contain, 
  at 
  their 
  top, 
  the 
  

   famous 
  Gloucestershire 
  " 
  Cephalopoda 
  beds," 
  and 
  do 
  not 
  yield 
  

   fossil 
  plants. 
  Furthermore, 
  they 
  are 
  recognized 
  as 
  transition 
  

   beds, 
  and 
  sometimes 
  actually 
  assigned 
  to 
  the 
  Lias. 
  

  

  Contrariwise, 
  as 
  will 
  presently 
  appear, 
  the 
  more 
  critical 
  

   comparison 
  of 
  the 
  Rajmahal 
  Hills 
  flora 
  with 
  that 
  of 
  

   Oaxaca 
  indicates 
  the 
  latter 
  to 
  be 
  essentially 
  juvenile, 
  there 
  

   being 
  distinct 
  reason 
  to 
  believe 
  that 
  as 
  the 
  study 
  of 
  this 
  

   prolific 
  region 
  for 
  fossil 
  plants 
  goes 
  on, 
  evidence 
  is 
  likely 
  to 
  

   increase 
  that 
  the 
  plants 
  in 
  the 
  lower 
  portion 
  of 
  the 
  beds 
  were 
  

   fossilized 
  soon 
  after 
  their 
  incursion 
  from 
  the 
  North 
  or 
  South, 
  

   or 
  soon 
  following 
  the 
  rapid 
  local 
  development 
  of 
  new 
  species. 
  

   At 
  least, 
  taking 
  the 
  Mesozoic 
  as 
  a 
  period 
  of 
  marked 
  changes 
  in 
  

   plant 
  facies, 
  there 
  must 
  be 
  a 
  certain 
  significance 
  in 
  the 
  persist- 
  

   ence 
  amongst 
  the 
  Oaxacan 
  plants 
  of 
  certain 
  old 
  elements, 
  

   which 
  in 
  longer 
  established 
  florae 
  might 
  be 
  mostly 
  eliminated. 
  

  

  Admittedly, 
  however, 
  the 
  conclusion 
  here 
  reached 
  can 
  only 
  

   have 
  a 
  tentative 
  value, 
  since 
  in 
  the 
  absence 
  of 
  inherent 
  evi- 
  

   dence 
  of 
  geologic 
  age 
  such 
  as 
  many 
  types 
  of 
  Jurassic 
  plants 
  

   fail 
  to 
  reveal 
  in 
  the 
  present 
  state 
  of 
  our 
  knowledge, 
  it 
  may 
  

   even 
  prove 
  uncertain 
  whether 
  given 
  groups 
  of 
  plants 
  which 
  

   appear 
  most 
  alike, 
  are 
  of 
  the 
  same 
  age 
  or 
  not. 
  All 
  that 
  can 
  

   be 
  done 
  safely 
  is 
  to 
  surmise 
  or 
  approximate. 
  For 
  not 
  only 
  are 
  

   the 
  chances 
  in 
  all 
  cases 
  very 
  great, 
  that 
  any 
  two 
  such 
  widely 
  

   separated 
  florae 
  are 
  not 
  exactly 
  synchronous; 
  but 
  however 
  

   strong 
  their 
  resemblance, 
  there 
  is 
  this 
  ever-present 
  possibility 
  

   that 
  one 
  is 
  old 
  and 
  long 
  established, 
  the 
  other 
  young 
  or 
  

   recently 
  established, 
  and 
  undergoing 
  change. 
  The 
  lack 
  of 
  

   various 
  old 
  types 
  in 
  the 
  first 
  case 
  and 
  their 
  retention 
  in 
  the 
  

   second 
  would, 
  of 
  course, 
  give 
  a 
  clear 
  result 
  if 
  we 
  knew 
  the 
  

   Jurassic 
  plant 
  succession 
  better. 
  In 
  general 
  it 
  seems 
  clear 
  that 
  

   the 
  pulsations 
  or 
  w 
  T 
  aves 
  of 
  plant 
  evolution 
  marking 
  successive 
  

   epochs 
  must 
  have 
  had 
  their 
  dawn, 
  their 
  high 
  noon, 
  and 
  their 
  

   eventide, 
  and 
  that 
  extinction, 
  in 
  any 
  period, 
  must 
  also 
  have 
  had 
  

   its 
  initial, 
  maximum, 
  and 
  minimum 
  phases 
  ; 
  though 
  it 
  is 
  even 
  

   clearer 
  that, 
  locally, 
  the 
  problems 
  of 
  plant 
  age 
  and 
  dispersion 
  

   soon 
  pass 
  beyond 
  the 
  known 
  facts 
  of 
  homotaxy, 
  and 
  that 
  they 
  

   involve 
  factors 
  of 
  the 
  utmost 
  difficulty. 
  

  

  But 
  without 
  further 
  outlining 
  these 
  obviously 
  severe 
  limi- 
  

   tations 
  to 
  accuracy 
  in 
  assigning 
  the 
  age 
  of 
  fossil 
  plants, 
  it 
  will 
  

   be 
  most 
  convenient, 
  as 
  a 
  prelude 
  to 
  such 
  discussion 
  of 
  the 
  age 
  

   and 
  source 
  of 
  the 
  Oaxacan 
  plants 
  as 
  can 
  be 
  given, 
  to 
  first 
  range 
  

   the 
  species 
  alongside 
  their 
  nearest 
  old 
  world 
  counterparts, 
  as 
  

   is 
  done 
  in 
  the 
  appended 
  Table 
  I. 
  

  

  