﻿GENERAL 
  CAUSES 
  OF 
  MIGRATION. 
  105 
  

  

  avoid 
  the 
  importunities 
  of 
  the 
  males, 
  after 
  repeated 
  short 
  flights, 
  at 
  

   length 
  riso 
  up 
  and 
  float 
  away 
  with 
  the 
  wind. 
  

  

  Our 
  only 
  comment 
  upon 
  this 
  eccentric 
  writer's 
  view 
  is, 
  that 
  a 
  careful 
  

   count 
  of 
  a 
  large 
  number 
  of 
  specimens 
  of 
  C. 
  sprctus 
  in 
  different 
  collections 
  

   shows 
  no 
  marked 
  difference 
  in 
  the 
  number 
  of 
  one 
  sex 
  as 
  compared 
  with 
  

   the 
  other. 
  In 
  our 
  First 
  Report 
  this 
  subject 
  was 
  alluded 
  to 
  207 
  , 
  and 
  we 
  see 
  

   no 
  reason 
  to 
  modify 
  what 
  is 
  there 
  stated. 
  

  

  Keferstein 
  remarks 
  208 
  that 
  — 
  

  

  The 
  same 
  causes 
  which 
  impel 
  the 
  Gryllus 
  italicus 
  to 
  take 
  the 
  field, 
  move 
  also 
  the 
  

   Gryllus 
  migratorius 
  to 
  his 
  wanderings, 
  namely, 
  want 
  of 
  food; 
  the 
  instinct 
  of 
  breeding, 
  

   which 
  the 
  female, 
  well 
  knowing 
  that 
  thereby 
  her 
  life-aim 
  will 
  be 
  accomplished, 
  and 
  

   she 
  must 
  then 
  die, 
  believes 
  will 
  be 
  accomplished 
  through 
  the 
  migration 
  ; 
  and, 
  finally, 
  

   the 
  instinct 
  to 
  seek 
  out 
  a 
  suitable 
  place 
  of 
  abode 
  for 
  their 
  progeny. 
  So 
  then 
  the 
  flight 
  

   of 
  all 
  species 
  of 
  locusts 
  may 
  be 
  said 
  to 
  depend 
  upon 
  similar 
  causes 
  at 
  bottom 
  : 
  neces- 
  

   sity, 
  love, 
  and 
  instinct 
  drive 
  them 
  thereto. 
  

  

  That 
  the 
  desire 
  to 
  seek 
  places 
  to 
  deposit 
  their 
  eggs 
  in 
  localities 
  which 
  

   are 
  best 
  adapted 
  to 
  the 
  young, 
  influences 
  locusts 
  as 
  well 
  as 
  other 
  insects, 
  

   cannot 
  be 
  doubted 
  ; 
  and 
  that 
  this 
  is 
  one 
  cause 
  of 
  migration 
  is 
  more 
  than 
  

   probable. 
  But 
  this 
  will 
  not 
  account 
  for 
  the 
  fact 
  that 
  they 
  are 
  essentially 
  

   migratory 
  even 
  in 
  their 
  native 
  habitats. 
  Xor 
  is 
  there 
  any 
  reason, 
  so 
  far 
  

   as 
  we 
  know, 
  why 
  it 
  should 
  operate 
  in 
  one 
  species 
  of 
  Acrididce 
  more 
  than 
  

   another 
  when 
  placed 
  under 
  the 
  same 
  circumstances. 
  (Edipoda 
  Carolina 
  

   is 
  found 
  throughout 
  the 
  area 
  occupied 
  by 
  C. 
  sprctus, 
  and 
  has 
  ample 
  wings 
  

   to 
  assist 
  it 
  in 
  flight, 
  but 
  it 
  is 
  never 
  found 
  migrating 
  in 
  any 
  true 
  sense 
  ; 
  

   yet 
  the 
  maternal 
  solicitude 
  for 
  the 
  welfare 
  of 
  the 
  young 
  is 
  doubtless 
  as 
  

   strong 
  in 
  the 
  one 
  species 
  as 
  the 
  other. 
  Neither 
  of 
  the 
  reasons 
  given, 
  

   therefore, 
  appears 
  to 
  be 
  satisfactory. 
  

  

  Darwin's 
  opinion, 
  as 
  already 
  quoted, 
  appears 
  to 
  be, 
  that 
  excessive 
  

   heat 
  causes 
  a 
  kind 
  of 
  irritation 
  or 
  uneasy 
  feeling 
  in 
  the 
  locusts, 
  which 
  

   makes 
  them 
  restless 
  and 
  desirous 
  of 
  seeking 
  some 
  place 
  where 
  they 
  can 
  

   be 
  sheltered 
  from 
  t 
  he 
  rays 
  of 
  the 
  sun. 
  That 
  some 
  kind 
  of 
  irritation 
  caused 
  

   by 
  excessive 
  heat 
  and 
  dryness 
  may 
  render 
  them 
  restless 
  and 
  uneasy, 
  is 
  

   not 
  only 
  possible 
  but 
  probable 
  ; 
  but 
  that 
  this 
  causes 
  them 
  to 
  fly 
  in 
  search 
  

   of 
  the 
  shelter 
  of 
  hills 
  and 
  forests 
  is 
  very 
  questionable, 
  especially 
  when 
  

   we 
  take 
  into 
  consideration, 
  as 
  heretofore 
  suggested, 
  the 
  well-known 
  fact 
  

   that, 
  as 
  an 
  almost 
  universal 
  rule, 
  they 
  avoid 
  forests 
  and 
  forest-clad 
  areas. 
  

  

  The 
  only 
  certain- 
  fact, 
  then, 
  that 
  we 
  have 
  to 
  start 
  with 
  in 
  our 
  investiga- 
  

   tion 
  as 
  to 
  the 
  origin 
  of 
  the 
  migratory 
  impulse 
  is, 
  that 
  it 
  is 
  in 
  some 
  way 
  

   connected 
  with 
  a 
  more 
  than 
  ordinarily 
  dry 
  and 
  rarefied 
  condition 
  of 
  the 
  

   atmosphere. 
  If 
  we 
  suppose 
  this 
  impulse 
  or 
  instinct 
  to 
  be 
  once 
  formed 
  

   by 
  such 
  climatic 
  conditions, 
  we 
  can 
  then 
  easily 
  explain 
  the 
  flights 
  in 
  the 
  

   lower 
  plains 
  of 
  the 
  temporary 
  region 
  where 
  the 
  atmosphere 
  is 
  more 
  dense 
  

   and 
  humid. 
  These 
  conditions, 
  combined 
  with 
  unusual 
  heat, 
  may 
  pro- 
  

   duce 
  a 
  kind 
  of 
  irritation 
  in 
  the 
  air-tubes, 
  and 
  this 
  may 
  cause 
  them 
  to 
  

   arise 
  in 
  search 
  of 
  a 
  more 
  rarefied 
  condition 
  of 
  the 
  air 
  ; 
  the 
  moving 
  breeze 
  

   is 
  found 
  to 
  favor 
  respiration 
  and 
  tends 
  to 
  allay 
  the 
  irritation 
  or 
  uneasy 
  

  

  *> 
  7 
  Page 
  250. 
  **Loc.cit. 
  

  

  