﻿270 
  AN-JSrUAL 
  EEPORT 
  SMITHSONIAN" 
  INSTITUTION, 
  192 
  9 
  

  

  like 
  many 
  another 
  fish, 
  it 
  offsets 
  its 
  unavoidable 
  loss 
  by 
  a 
  greater 
  pro- 
  

   duction 
  of 
  eggs. 
  Carnivorous 
  animals 
  and 
  their 
  prey 
  inhabit 
  the 
  same 
  

   territory 
  for 
  an 
  indefinite 
  time, 
  but 
  a 
  balance 
  in 
  their 
  proportionate 
  

   numbers 
  is 
  maintained. 
  

  

  As 
  a 
  matter 
  of 
  fact, 
  the 
  organic 
  world 
  of 
  the 
  present 
  time, 
  both 
  of 
  

   animals 
  and 
  plants, 
  in 
  sea 
  and 
  on 
  land, 
  maintains 
  a 
  balance 
  so 
  perfect 
  

   that 
  it 
  is 
  brought 
  to 
  our 
  attention 
  only 
  when 
  disturbed 
  by 
  the 
  inter- 
  

   vention 
  of 
  man. 
  As 
  between 
  two 
  animals, 
  this 
  balance 
  operates 
  for 
  

   the 
  benefit 
  of 
  both, 
  for 
  the 
  extermination 
  of 
  the 
  weaker 
  by 
  the 
  stronger 
  

   would 
  mean 
  the 
  destruction 
  of 
  the 
  source 
  of 
  food 
  of 
  the 
  stronger, 
  re- 
  

   sulting 
  in 
  its 
  extermination 
  also. 
  If 
  the 
  Silurian 
  and 
  Devonian 
  fishes 
  

   could 
  have 
  completely 
  exterminated 
  the 
  trilobites, 
  they 
  would 
  prob- 
  

   ably 
  have 
  doomed 
  their 
  own 
  existence. 
  Moreover, 
  the 
  extinction 
  of 
  

   the 
  trilobites 
  was 
  not 
  catastrophic; 
  it 
  was 
  in 
  operation 
  during 
  the 
  

   Silurian 
  period, 
  when 
  their 
  decline 
  is 
  first 
  noticeable, 
  and 
  it 
  continued 
  

   through 
  the 
  Devonian 
  and 
  Carboniferous 
  periods, 
  or, 
  speaking 
  in 
  terms 
  

   of 
  years, 
  through 
  many 
  decades 
  of 
  millions 
  of 
  years 
  — 
  a 
  time 
  long 
  enough 
  

   to 
  permit 
  the 
  establishment 
  and 
  maintenance 
  of 
  the 
  natural 
  balance. 
  

   Then, 
  also, 
  the 
  details 
  of 
  their 
  dying 
  out 
  do 
  not 
  support 
  the 
  explana- 
  

   tion 
  given. 
  In 
  Silurian 
  time 
  the 
  number 
  of 
  individual 
  trilobites 
  was 
  

   abundant, 
  but 
  the 
  number 
  of 
  genera 
  and 
  species 
  as 
  compared 
  with 
  

   those 
  in 
  the 
  Cambrian 
  and 
  Ordovician 
  had 
  been 
  greatly 
  reduced.^ 
  

   Moreover, 
  they 
  developed 
  a 
  number 
  of 
  features, 
  such 
  as 
  spines, 
  pro- 
  

   tuberances, 
  and 
  enlargements 
  of 
  parts, 
  which 
  primarily 
  serve, 
  no 
  

   doubt, 
  as 
  protective 
  devices,^ 
  but 
  some 
  of 
  which, 
  by 
  their 
  extreme 
  

   enlargement, 
  eventually 
  lost 
  their 
  protective 
  value. 
  The 
  enlarge- 
  

   ment 
  of 
  certain 
  features 
  was 
  common 
  to 
  many 
  extinct 
  animals, 
  and 
  it 
  

   might 
  be 
  considered 
  a 
  proof 
  of 
  their 
  racial 
  old 
  age.^ 
  It 
  appears 
  to 
  be 
  

   a 
  result 
  of 
  the 
  heroic 
  efforts 
  of 
  a 
  race 
  to 
  maintain 
  an 
  organic 
  stock 
  that 
  

   is 
  losing 
  its 
  vitality.® 
  Such 
  efforts 
  were 
  made 
  by 
  the 
  trilobites 
  mil- 
  

   lions 
  of 
  years 
  before 
  they 
  were 
  serioulsy 
  menaced 
  by 
  their 
  enemies. 
  

   These 
  peculiar 
  features 
  of 
  organization 
  were 
  not 
  the 
  results 
  of 
  the 
  

   attack 
  of 
  the 
  fishes, 
  but 
  were 
  due 
  to 
  causes 
  within 
  the 
  trilobites 
  them- 
  

   selves. 
  The 
  trilobites 
  may 
  have 
  disappeared 
  only 
  because 
  as 
  a 
  race 
  

   they 
  had 
  become 
  old, 
  had 
  lost 
  their 
  vitality, 
  and 
  were 
  unable 
  to 
  estab- 
  

   lish 
  and 
  maintain 
  the 
  natural 
  balance. 
  

  

  THE 
  MESOZOIC 
  REPTILES 
  

  

  Nor 
  can 
  the 
  extinction 
  of 
  Mesozoic 
  reptiles 
  and 
  gigantic 
  dinosaurs 
  

   be 
  explained 
  satisfactorily. 
  The 
  animals 
  dominated 
  the 
  earth 
  more 
  

   completely 
  than 
  do 
  the 
  mammals 
  of 
  to-day, 
  and 
  certainly 
  they 
  had 
  

  

  ' 
  Charles 
  Schuchert, 
  Historical 
  geology, 
  p. 
  210. 
  

   « 
  Idem, 
  p. 
  271. 
  

   ' 
  Idem, 
  p. 
  11. 
  

   » 
  Idem, 
  p. 
  210. 
  

  

  