﻿G. 
  R. 
  Wieland 
  — 
  On 
  Liassic 
  Floras. 
  281 
  

  

  cursor 
  of 
  either 
  northern 
  or 
  southern 
  floras, 
  appears 
  entirely 
  

   gratuitous. 
  The 
  lack 
  of 
  the 
  cosmopolitan 
  Dictyozamites, 
  

   together 
  with 
  the 
  apparent 
  absence 
  of 
  conifers 
  and 
  Ginkgos, 
  

   points 
  the 
  other 
  way. 
  The 
  general 
  movement 
  of 
  plants 
  in 
  

   more 
  recent 
  time 
  is 
  also 
  against 
  such 
  a 
  view 
  ; 
  and 
  it 
  is 
  perti- 
  

   nent 
  to 
  iterate 
  here 
  that 
  it 
  has 
  too 
  long 
  been 
  the 
  custom 
  of 
  

   paleontologists 
  when 
  comparing 
  the 
  fossils 
  of 
  remote 
  horizons 
  

   to 
  imagine 
  that 
  the 
  similarities 
  observed 
  are 
  due 
  to 
  some 
  con- 
  

   stant 
  interchange 
  of 
  species, 
  originating 
  locally 
  and 
  more 
  or 
  

   less 
  by 
  chance. 
  Too 
  often 
  this 
  idea 
  of 
  the 
  crossing 
  of 
  species 
  

   and 
  the 
  recrossing 
  half 
  way 
  round 
  the 
  globe 
  is 
  hypothesized 
  

   in 
  terms 
  excluding 
  the 
  polar 
  areas 
  ; 
  for 
  it 
  is 
  only 
  reasonable 
  to 
  

   suppose 
  that 
  these 
  have 
  always 
  been 
  relatively 
  more 
  instead 
  of 
  

   less 
  prolific 
  of 
  new 
  species 
  than 
  equatorial 
  regions. 
  

  

  Unquestionably, 
  when 
  isolated 
  localities 
  are 
  freshly 
  popu- 
  

   lated, 
  especially 
  islands, 
  considerable 
  specific 
  variation 
  results. 
  

   But 
  it 
  is 
  a 
  fair 
  inference 
  that 
  species 
  so 
  produced 
  have 
  much 
  

   less 
  invasive 
  power 
  than 
  those 
  which 
  result 
  from 
  pro 
  founder 
  

   geologic 
  changes 
  affecting 
  the 
  globe 
  as 
  a 
  unit 
  — 
  or 
  better 
  said, 
  

   perhaps, 
  those 
  species 
  which 
  mark 
  and 
  form 
  the 
  crests 
  of 
  the 
  

   greater 
  waves 
  of 
  evolutionary 
  development 
  and 
  change. 
  It 
  is, 
  

   therefore, 
  a 
  strong 
  inference 
  from 
  the 
  general 
  as 
  well 
  as 
  special 
  

   facts 
  cited 
  that 
  the 
  Oaxacan 
  flora, 
  though 
  no 
  doubt 
  including 
  

   many 
  forms 
  or 
  varieties 
  of 
  local 
  development, 
  was 
  preponder- 
  

   antly 
  northern 
  rather 
  than 
  southern 
  or 
  mainly 
  equatorial 
  in 
  

   aspect. 
  

  

  The 
  very 
  few 
  generalizations 
  tentatively 
  outlined 
  here 
  must, 
  

   of 
  course, 
  await 
  the 
  results 
  of 
  future 
  field 
  work 
  for 
  their 
  proof 
  

   or 
  disproof. 
  Nevertheless, 
  it 
  does 
  seem 
  that 
  the 
  overlap 
  of 
  

   new 
  and 
  old 
  forms 
  in 
  the 
  various 
  Jurassic 
  florae 
  can 
  be 
  satis- 
  

   factorily 
  dealt 
  with 
  as 
  soon 
  as 
  species 
  are 
  better 
  known. 
  The 
  

   strong 
  likeness 
  between 
  such 
  widely 
  separated 
  florae 
  as 
  those 
  

   just 
  considered 
  betokens 
  regularity 
  in 
  the 
  movement 
  and 
  

   development 
  of 
  Jurassic 
  plant 
  life. 
  It 
  is, 
  therefore, 
  nearly 
  

   certain 
  that 
  while 
  it 
  may 
  never 
  be 
  possible 
  to 
  trace 
  the 
  full 
  

   history 
  of 
  genera 
  or 
  families 
  one 
  after 
  the 
  other, 
  the 
  accumu- 
  

   lation 
  of 
  large 
  aggregates 
  of 
  definitely 
  determined 
  species 
  will 
  

   be 
  equally 
  effective. 
  Aggregates 
  of 
  species 
  should 
  enable 
  us 
  

   to 
  determine 
  age 
  with 
  accuracy 
  on 
  the 
  basis 
  of 
  the 
  percentage 
  

   of 
  the 
  major 
  elements, 
  that 
  is 
  the 
  Cordaitaleans, 
  cycads, 
  con- 
  

   ifers, 
  ferns 
  and 
  Equisetums, 
  as 
  approximated 
  on 
  Table 
  Y. 
  

   Certainly 
  it 
  seems 
  that 
  eventually 
  it 
  should 
  be 
  possible 
  to 
  

   establish 
  the 
  curve 
  of 
  frequency 
  for 
  the 
  orders 
  of 
  plants 
  in 
  

   the 
  several 
  florae, 
  however 
  erratic 
  may 
  have 
  been 
  the 
  devel- 
  

   opment 
  and 
  spread 
  of 
  some 
  of 
  the 
  genera 
  or 
  families. 
  And 
  

   so 
  long 
  as 
  such 
  possibility 
  remains 
  open, 
  the 
  incentive 
  to 
  the 
  

   accurate 
  determination 
  of 
  the 
  Jurassic 
  species 
  is 
  of 
  the 
  strongest. 
  

  

  