﻿404 
  G. 
  R. 
  Wieland 
  on 
  the 
  Cycadophyta. 
  

  

  ance 
  of 
  typical 
  forest 
  trees, 
  groups 
  or 
  phyla 
  ; 
  and 
  where 
  

   there 
  is 
  comparative 
  evidence 
  for 
  earlier 
  origin 
  than 
  the 
  

   fossil 
  record 
  shows, 
  the 
  fact 
  is 
  in 
  a 
  general 
  way 
  indicated. 
  

   But 
  such 
  a 
  scheme 
  is 
  always 
  provisional. 
  The 
  upturned 
  

   lines 
  are 
  used 
  because, 
  as 
  well 
  known, 
  the 
  study 
  of 
  inver- 
  

   tebrates, 
  vertebrates 
  and 
  plants 
  shows 
  that 
  when 
  new 
  

   types 
  first 
  appear, 
  the 
  evolution 
  of 
  species 
  is 
  rapid, 
  and 
  

   the 
  later 
  decline 
  slow. 
  

  

  Secondly, 
  the 
  terminal 
  gymnosperm 
  lines 
  show 
  an 
  

   inherent 
  relationship 
  so 
  far 
  back 
  in 
  time 
  that 
  it 
  is 
  not 
  

   always 
  consistent 
  to 
  speak 
  of 
  these 
  more 
  specialized 
  ter- 
  

   mini 
  as 
  absolutely 
  extinct. 
  They 
  are 
  only 
  relatively 
  so. 
  

   The 
  record 
  proves 
  rather 
  that 
  the 
  forward 
  movement 
  was 
  

   always 
  widespread, 
  with 
  the 
  extinct 
  forms 
  always 
  struc- 
  

   turally 
  near 
  the 
  persisting 
  mean. 
  Specialization 
  rises 
  

   little 
  beyond 
  oddity 
  of 
  outer 
  feature. 
  Of 
  this 
  the 
  Gink- 
  

   gos 
  are 
  a 
  notable 
  instance. 
  

  

  Those 
  who 
  work 
  much 
  with 
  the 
  recent 
  types, 
  or 
  those 
  

   relatively 
  recent, 
  like 
  the 
  mammalia, 
  may 
  be 
  led 
  to 
  

   regard 
  evolution 
  as 
  one 
  vast 
  series 
  of 
  adaptations 
  and 
  

   specializations, 
  and 
  thus 
  become 
  ardent 
  culturists 
  of 
  

   paleontologic 
  trees. 
  Such 
  must 
  ever 
  express 
  the 
  facts 
  

   too 
  superficially; 
  the 
  main 
  or 
  greater 
  course 
  of 
  change 
  

   is 
  far 
  better 
  defined 
  as 
  one 
  of 
  modification, 
  reduction, 
  and 
  

   the 
  avoidance 
  of 
  specialization. 
  

  

  The 
  exceedingly 
  long 
  plant 
  series 
  permits 
  a 
  glimpse 
  of 
  

   the 
  average 
  course 
  of 
  upward 
  change, 
  and 
  the 
  great 
  

   role 
  of 
  reductions. 
  Its 
  study, 
  so 
  often 
  severely 
  histologic, 
  

   also 
  brings 
  into 
  closer 
  view 
  the 
  climatic 
  factors. 
  But 
  

   the 
  study 
  is 
  only 
  just 
  begun. 
  Hence 
  the 
  introduction 
  of 
  

   the 
  brief 
  climate 
  characterizations 
  in 
  diagram 
  2, 
  is 
  only 
  

   meant 
  to 
  emphasize 
  the 
  fact 
  that 
  there 
  is 
  continuity 
  in 
  the 
  

   changes 
  going 
  on 
  from 
  age 
  to 
  age. 
  Geologists 
  may 
  even 
  

   object 
  to 
  these 
  terms, 
  — 
  in 
  that 
  case 
  because 
  the 
  compila- 
  

   tion 
  is 
  faulty. 
  [The 
  continental 
  area 
  curve 
  is 
  from 
  

   Schuchert.] 
  Similarly, 
  terms 
  correspondent 
  to 
  the 
  

   Siluro-Devonic, 
  Carboniferous, 
  Permo-Triassic, 
  and 
  

  

  6 
  In 
  bringing 
  climate 
  into 
  the 
  time 
  scheme, 
  the 
  main 
  uncertain 
  factors 
  

   are 
  cloudiness 
  and 
  atmospheric 
  composition, 
  both 
  unmeasurable. 
  Climate 
  

   and 
  diastrophism 
  are, 
  however, 
  inseparable 
  factors, 
  whether 
  in 
  dealing 
  with 
  

   successive 
  florae 
  precise 
  inferences 
  can 
  at 
  once 
  be 
  gained 
  or 
  not. 
  Certain 
  

   periods 
  have 
  been 
  cold 
  and 
  arid, 
  others 
  wetter 
  in 
  both 
  time 
  and 
  space. 
  A 
  

   continent-wide 
  soil 
  change 
  is 
  thus 
  also 
  ever 
  going 
  far 
  to 
  control 
  forest 
  

   limits 
  and 
  tvpes. 
  With 
  the 
  appearance 
  of 
  bipolar 
  ice 
  caps 
  a 
  kind 
  of 
  

   culmination 
  is 
  reached. 
  These 
  are 
  the 
  greater 
  factors 
  which 
  led 
  during 
  

   transition 
  time 
  for 
  instance 
  to 
  the 
  specialized 
  stem 
  of 
  an 
  oak, 
  one 
  of 
  the 
  

   highest 
  plant 
  products 
  yet 
  seen. 
  

  

  