﻿CAUSES 
  OF 
  MIGKATION. 
  103 
  

  

  planation 
  as 
  the 
  most 
  satisfactory. 
  Our 
  native 
  species, 
  instead 
  of 
  seek- 
  

   ing 
  forests 
  as 
  a 
  place 
  of 
  shelter, 
  appear 
  to 
  avoid 
  them 
  as 
  far 
  as 
  possi- 
  

   ble. 
  

  

  Darwin's 
  theory 
  is 
  undoubtedly 
  contradicted 
  by 
  the 
  characteristics 
  

   of 
  all 
  the 
  migratory 
  species. 
  

  

  8. 
  THE 
  CAUSES 
  OF 
  MIGRATION. 
  

  

  As 
  stated 
  in 
  our 
  First 
  Keport, 
  206 
  "we 
  must 
  recognize 
  the 
  fact 
  that 
  the 
  

   influences 
  bearing 
  on 
  migration 
  fall 
  into 
  two 
  distinct 
  categories, 
  viz, 
  

   remote 
  or 
  general, 
  and 
  immediate 
  or 
  special." 
  We 
  shall 
  therefore 
  briefly 
  

   consider 
  the 
  two 
  classes 
  separately. 
  207 
  

  

  a, 
  Remote 
  causes. 
  — 
  Why 
  one 
  species 
  of 
  insect 
  should 
  at 
  certain 
  times 
  

   develop 
  in 
  immense 
  numbers 
  and 
  migrate, 
  while 
  another 
  closely 
  allied 
  

   species, 
  inhabiting 
  the 
  same 
  locality, 
  and 
  to 
  all 
  appearance 
  subject 
  to 
  

   the 
  same 
  influences, 
  never 
  increases 
  to 
  the 
  same 
  extent 
  and 
  never 
  exhib- 
  

   its 
  a 
  disposition 
  to 
  migrate, 
  is 
  a 
  question 
  which 
  has 
  long 
  puzzled 
  ento- 
  

   mologists. 
  

  

  If 
  entomologists 
  are 
  asked 
  why 
  locusts 
  migrate, 
  ninety-nine 
  out 
  of 
  

   every 
  hundred 
  will 
  probably 
  answer, 
  "On 
  account 
  of 
  excessive 
  numhers? 
  

   We 
  might 
  press 
  the 
  inquiry 
  farther 
  and 
  ask 
  why 
  they 
  develop 
  in 
  such 
  

   excessive 
  numbers. 
  But 
  we 
  propose, 
  at 
  present, 
  to 
  seek 
  for 
  the 
  remote 
  

   causes 
  by 
  another 
  road, 
  as 
  the 
  attempt 
  to 
  press 
  back 
  the 
  inquiry 
  step 
  

   by 
  step 
  would 
  lead 
  us 
  into 
  a 
  labyrinth 
  of 
  biological 
  questions 
  which 
  we 
  

   have 
  no 
  occasion 
  to 
  enter 
  upon 
  at 
  this 
  time. 
  

  

  A 
  thorough 
  and 
  exhaustive 
  examination 
  of 
  this 
  question 
  would 
  carry 
  

   us 
  back 
  into 
  the 
  last 
  geological 
  changes 
  of 
  the 
  earth's 
  surface; 
  but 
  this 
  

   we 
  have 
  not 
  the 
  time 
  to 
  undertake 
  if 
  we 
  felt 
  qualified 
  to 
  do 
  so, 
  which 
  

   we 
  by 
  no 
  means 
  claim. 
  

  

  As 
  we 
  have 
  heretofore 
  shown, 
  migratory 
  locusts 
  are 
  found 
  only 
  in 
  tree- 
  

   less, 
  dry, 
  and 
  more 
  or 
  less 
  barren 
  regions, 
  and, 
  as 
  a 
  very 
  general 
  rule, 
  

   their 
  breeding 
  grounds 
  are 
  in 
  areas 
  or 
  plateaus 
  of 
  considerable 
  eleva 
  : 
  

   tion. 
  The 
  native 
  home, 
  for 
  example, 
  of 
  P. 
  migratorius, 
  appears 
  to 
  have 
  

   been 
  the 
  steppes 
  of 
  Southern 
  Siberia 
  and 
  Tartary. 
  J., 
  peregrinum 
  has 
  

   its 
  points 
  of 
  greatest 
  development 
  in 
  Central 
  Arabia 
  and 
  the 
  dry, 
  ele- 
  

   vated 
  table-lands 
  of 
  Northern 
  Africa; 
  A. 
  paranense, 
  in 
  the 
  higher 
  bar- 
  

   ren 
  plateaus 
  of 
  Argentine 
  Republic; 
  and 
  C. 
  spretus, 
  in 
  the 
  high, 
  barren 
  

   regions 
  of 
  the 
  Rocky 
  Mountains, 
  and 
  the 
  elevated 
  plains 
  of 
  Montana, 
  

   Western 
  Dakota, 
  and 
  British 
  America. 
  The 
  marked 
  characteristics 
  of 
  

   all 
  these 
  regions 
  are, 
  absence 
  of 
  forests, 
  more 
  than 
  ordinary 
  dryness, 
  and 
  

   rarefied 
  air. 
  Keferstein 
  remarks 
  that 
  — 
  

  

  A 
  dry. 
  warm, 
  uncultivated, 
  treeless 
  plain, 
  where 
  the 
  brood 
  can 
  be 
  deposited 
  un- 
  

   disturbed 
  and 
  left 
  to 
  gntyw 
  up, 
  is 
  especially 
  favorable 
  to 
  the 
  propagation 
  of 
  the 
  locust, 
  

   and 
  in 
  such 
  districts 
  of 
  country 
  the 
  locust 
  plague 
  appears 
  most 
  frequently 
  and 
  regu- 
  

  

  206 
  Page 
  249. 
  

  

  ^Itis 
  proper 
  to 
  state 
  here 
  that 
  -while 
  the 
  Commissioners 
  agree 
  in 
  the 
  main 
  as 
  to 
  the 
  causes 
  of 
  migra 
  

   tion 
  the 
  writer 
  of 
  this 
  chapter 
  must 
  he 
  held 
  alone 
  responsible 
  for 
  some 
  of 
  the 
  views 
  advanced 
  here. 
  

  

  