﻿IMMEDIATE 
  CAUSES 
  OF 
  MIGRATION. 
  107 
  

  

  shrink 
  or 
  lessen 
  the 
  size 
  of 
  the 
  softer 
  parts. 
  As 
  a 
  very 
  general 
  rule, 
  more- 
  

   species 
  of 
  the 
  (Edipoid 
  groups 
  are 
  found 
  in 
  barren 
  areas 
  than 
  of 
  the 
  Acri- 
  

   didae; 
  the 
  Acridida?, 
  on 
  the 
  other 
  hand, 
  are 
  generally 
  fleshier 
  or, 
  to 
  use 
  a 
  

   botanical 
  term, 
  more 
  succulent 
  than 
  the 
  (Edipodse 
  ; 
  it 
  follows, 
  therefore, 
  

   that 
  the 
  effect 
  of 
  unusual 
  dryness 
  would 
  manifest 
  itself 
  much 
  sooner 
  in 
  

   the 
  Acridians 
  than 
  in 
  the 
  CEdipodeans, 
  and 
  such 
  is 
  undoubtedly 
  the 
  

   fact. 
  Although 
  we 
  are 
  unable 
  to 
  follow 
  out 
  fully 
  the 
  effect 
  of 
  the 
  dry 
  

   condition 
  of 
  the 
  atmosphere 
  on 
  the 
  locust 
  system 
  and 
  tell 
  just 
  how 
  and 
  

   why 
  it 
  results 
  in 
  the 
  migratory 
  habit, 
  yet 
  we 
  are 
  satisfied 
  we 
  are 
  now 
  on 
  

   the 
  direct 
  course 
  toward 
  and 
  very 
  near 
  the 
  solution 
  of 
  the 
  question. 
  

   The 
  usually 
  slender 
  form 
  and 
  unusually 
  hard 
  and 
  horny 
  condition 
  of 
  

   the 
  external 
  crust 
  agree 
  with 
  this 
  theory, 
  as 
  does 
  also 
  the 
  tendency 
  of 
  

   the 
  ultimate 
  sternite 
  of 
  the 
  male 
  (sub-anal 
  plate) 
  to 
  grow 
  narrower 
  and 
  

   become 
  elongated. 
  During 
  the 
  long-continued 
  dry 
  seasons 
  of 
  1874-187(> 
  

   the 
  effect 
  was 
  very 
  visible 
  on 
  Caloptenus 
  differ 
  entialis, 
  a 
  species 
  which 
  

   delights 
  in 
  rank 
  vegetation, 
  and 
  is, 
  if 
  possible, 
  more 
  lubberly 
  than 
  C~ 
  

   bivittatus. 
  Examining 
  specimens 
  of 
  a 
  swarm 
  that 
  had 
  flown 
  from 
  Ken- 
  

   tucky 
  and 
  had 
  fallen 
  in 
  the 
  streets 
  of 
  Cairo, 
  111., 
  in 
  large 
  numbers, 
  I 
  

   was 
  astonished 
  at 
  the 
  change 
  from 
  the 
  typical 
  form 
  ; 
  they 
  had 
  a 
  wild 
  r 
  

   ferocious 
  look 
  similar 
  to 
  that 
  observed 
  in 
  C. 
  spretus 
  ; 
  their 
  bodies 
  were 
  

   much 
  slenderer 
  than 
  usual 
  ; 
  there 
  was 
  a 
  compactness 
  or, 
  to 
  use 
  a 
  term 
  

   which 
  expresses 
  exactly 
  their 
  appearance, 
  a 
  "trimness" 
  of 
  form 
  never 
  

   before 
  observed 
  in 
  them. 
  

  

  If 
  the 
  theory 
  here 
  advanced 
  is 
  correct 
  — 
  and 
  it 
  is 
  really 
  but 
  a 
  slight 
  

   extension 
  of 
  an 
  opinion 
  long 
  maintained 
  and 
  generally 
  received 
  — 
  we 
  

   have 
  a 
  partial 
  explanation 
  at 
  least 
  of 
  the 
  reason 
  why 
  C. 
  spretus 
  became 
  

   migratory 
  and 
  the 
  CEdipodeans, 
  inhabiting 
  the 
  same 
  region, 
  did 
  not. 
  

   We 
  also 
  have 
  a 
  theory 
  which 
  agrees 
  with 
  all 
  the 
  data 
  bearing 
  upon 
  the 
  

   question 
  and 
  which 
  explains 
  most 
  of 
  the 
  facts 
  and 
  does 
  not 
  stand 
  in 
  

   opposition 
  to 
  any. 
  If 
  this 
  theory 
  is 
  correct, 
  and 
  we 
  are 
  not 
  to 
  suppose 
  

   a 
  species 
  was 
  formed 
  with 
  a 
  migratory 
  instinct, 
  then 
  it 
  follows 
  that 
  

   whatever 
  produced 
  or 
  brought 
  about 
  the 
  arid 
  condition 
  of 
  the 
  area 
  

   where 
  a 
  migrating 
  species 
  originated 
  or 
  became 
  migratory 
  was 
  the 
  ulti- 
  

   mate 
  cause 
  of 
  the 
  migratory 
  instinct. 
  

  

  As 
  the 
  arid 
  condition 
  of 
  the 
  mountain 
  region 
  and 
  plains 
  of 
  the 
  West 
  

   is, 
  in 
  a 
  geological 
  sense, 
  of 
  comparatively 
  recent 
  date, 
  I 
  think 
  it 
  more 
  

   than 
  probable 
  that 
  C. 
  spretus 
  is 
  but 
  a 
  modified 
  form 
  of 
  C.femur-rubrum; 
  

   and 
  that 
  several 
  of 
  the 
  apparently 
  local 
  species 
  found 
  in 
  Kansas,. 
  

   Nebraska, 
  Texas, 
  and 
  Minnesota, 
  which 
  are 
  closely 
  allied, 
  are, 
  so 
  to 
  

   speak, 
  but 
  results 
  of 
  the 
  visits 
  of 
  C. 
  spretus; 
  stragglers, 
  left 
  in 
  their 
  at- 
  

   tempt 
  to 
  return 
  to 
  their 
  native 
  habitats, 
  produce 
  a 
  second 
  brood 
  in 
  lim- 
  

   ited 
  numbers 
  and 
  a 
  combination 
  of 
  favorable 
  circumstances 
  preserve 
  

   them 
  from 
  the 
  usual 
  fate, 
  and 
  in 
  this 
  way 
  the 
  local 
  form 
  results. 
  

  

  Immediate 
  eanses. 
  — 
  If 
  the 
  theory 
  advanced 
  is 
  correct 
  there 
  is 
  no 
  neces- 
  

   sity 
  for 
  searching 
  after 
  immediate 
  causes, 
  as 
  they 
  are 
  essentially 
  or 
  we 
  

   might 
  say 
  constitutionally 
  migratory 
  and 
  will 
  take 
  to 
  flight 
  without 
  any 
  

  

  