﻿656 
  REPORT 
  UNITED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  

  

  very 
  bread, 
  of 
  temporary 
  visitations, 
  "whicli 
  is 
  to 
  the 
  area 
  properly 
  so 
  called 
  what 
  

   the 
  pennmbra 
  is 
  to 
  the 
  light, 
  within 
  the 
  zone, 
  of 
  which 
  the 
  exterior 
  limit 
  is 
  much 
  more 
  

   easy 
  to 
  trace 
  than 
  the 
  inner; 
  this 
  last 
  is 
  finbject 
  to 
  continnal 
  oscillations, 
  with 
  some 
  

   undulatory 
  movements, 
  dependent 
  on 
  the 
  centrifugal 
  or 
  expansive 
  tendency 
  of 
  the 
  

   species, 
  and 
  from 
  the 
  resistance 
  which 
  opposes 
  it, 
  and 
  external 
  circumstances, 
  and 
  evi- 
  

   dently 
  also 
  the 
  tendency 
  of 
  other 
  species 
  to 
  spread 
  out, 
  with 
  which 
  it 
  carries 
  on 
  a 
  

   struggle 
  for 
  existence 
  in 
  endeavoring 
  to 
  maintain 
  itself 
  on 
  an 
  earth 
  where 
  the 
  chances 
  

   are 
  divided, 
  and 
  even 
  vary 
  from 
  year 
  to 
  year. 
  M. 
  Koppen 
  has 
  thus 
  been 
  enabled 
  to 
  

   figure 
  on 
  his 
  chart 
  three 
  lines, 
  as 
  I 
  may 
  for 
  the 
  present 
  call 
  them, 
  and 
  the 
  intermediate 
  

   line 
  represents 
  the 
  exterior 
  actual 
  limit 
  of 
  these 
  oscillations 
  of 
  the 
  true 
  frontier 
  line 
  

   of 
  PachytyJus 
  migralorius 
  ; 
  their 
  amplitude 
  may 
  vary 
  from 
  two 
  to 
  four 
  degrees." 
  

  

  The 
  last 
  thesis 
  of 
  M. 
  Koppen 
  that 
  I 
  shall 
  draw 
  attention 
  to 
  at 
  this 
  time, 
  namely, 
  

   that 
  the 
  absence 
  of 
  Pacliyiylus 
  migraforius 
  in 
  America 
  should 
  prove 
  that 
  the 
  specie 
  i 
  

   exists 
  only 
  as 
  a 
  species, 
  since 
  the 
  separation 
  of 
  the 
  two 
  continents 
  toward 
  the 
  north 
  

   pole, 
  seems 
  to 
  me 
  scarcely 
  necessary. 
  A 
  mere 
  glance 
  at 
  the 
  map 
  which 
  represents 
  the 
  

   area 
  of 
  distribution 
  of 
  this 
  locust 
  allows 
  us 
  to 
  affirm 
  without 
  hesitation 
  that 
  that 
  view 
  

   is 
  impossible. 
  It 
  is 
  evidently 
  not 
  one 
  of 
  those 
  species 
  which 
  we 
  may 
  call 
  circumboreal 
  

   anieglacial, 
  because 
  their 
  presence 
  in 
  two 
  forms 
  (races, 
  varieties, 
  or 
  species) 
  on 
  each 
  

   continent 
  indicates 
  that 
  they 
  have 
  had 
  a 
  common 
  origin, 
  a 
  single 
  area 
  at 
  that 
  epoch, 
  

   anterior 
  to 
  the 
  glacial 
  period, 
  when 
  the 
  two 
  continents 
  were 
  reunited 
  in 
  the 
  Arctic 
  

   zone 
  by 
  a 
  bridge, 
  so 
  to 
  speak, 
  that 
  is, 
  a 
  continuity 
  of 
  land, 
  in 
  conditions 
  of 
  climate 
  

   which 
  should 
  allow 
  the 
  existence 
  at 
  that 
  latitude 
  of 
  a 
  fauna 
  which 
  only 
  at 
  present 
  

   exists 
  much 
  farther 
  south. 
  The 
  source 
  of 
  those 
  species 
  dispersed 
  by 
  the 
  glacial 
  period 
  

   does 
  not 
  now 
  probably 
  exist 
  in 
  its 
  integrity 
  ; 
  but 
  the 
  two 
  races 
  confined, 
  one 
  in 
  America, 
  

   the 
  other 
  in 
  the 
  Old 
  World, 
  having 
  undergone 
  slow 
  modifications 
  each 
  on 
  its 
  part, 
  are 
  

   to-day 
  very 
  analogous 
  species, 
  but 
  as 
  distinct 
  by 
  their 
  external 
  characters 
  as 
  by 
  their 
  

   separate 
  geographical 
  area. 
  

  

  Nothing 
  like 
  this 
  applies 
  to 
  PacJiytyhis 
  migraiorius 
  ; 
  it 
  is 
  one 
  of 
  those 
  species 
  which 
  

   may 
  be 
  called 
  equatorial 
  postglacial 
  ; 
  its 
  expansion 
  toward 
  the 
  north 
  has 
  been 
  posterior 
  

   to 
  the 
  glacial 
  period, 
  which 
  would 
  then 
  have 
  opposed 
  it 
  ; 
  and 
  it 
  can 
  have 
  no 
  affinities 
  

   in 
  the 
  New 
  World, 
  but 
  degrees 
  of 
  consanguinity 
  much 
  farther 
  removed 
  than 
  those 
  

   unite 
  the 
  circumboreal 
  species 
  of 
  the 
  temperate 
  zone. 
  Thus, 
  if, 
  as 
  sdme 
  think, 
  the 
  

   northern 
  hemisiihere 
  tends 
  actually 
  to 
  retrograde 
  toward 
  a 
  new 
  period 
  of 
  cold, 
  the 
  

   Pachytyhis 
  migraiorius 
  is 
  destined 
  to 
  see 
  its 
  area 
  also 
  retrograde 
  toward 
  the 
  equator, 
  

   and 
  perhaps 
  some 
  day 
  the 
  western 
  and 
  eastern 
  parts 
  of 
  this 
  area 
  may 
  be 
  completely 
  

   disjointed, 
  and, 
  following 
  this 
  separation, 
  its 
  posterity 
  may 
  be 
  so 
  modified 
  by 
  isolation 
  

   as 
  to 
  form 
  two 
  distinct 
  species, 
  as 
  has 
  occurred 
  to 
  circumpolar 
  species. 
  

  

  lu 
  the 
  discussion 
  which 
  followed, 
  M. 
  de 
  Sel.vs 
  Longchamps 
  speaks 
  of 
  

   the 
  difiiculty 
  of 
  Si^p?iTSitmg 
  Fachytylusmigratorius 
  (Liun.) 
  aud 
  cinerascens 
  

   (Fabr.), 
  which 
  he 
  had 
  at 
  first 
  regarded 
  as 
  varieties, 
  bat 
  now 
  considers 
  as 
  

   a 
  distinct 
  species, 
  the 
  latter 
  being 
  more 
  sedentary 
  and 
  reproducing 
  in 
  

   Belginm 
  year 
  after 
  year 
  : 
  " 
  M. 
  F. 
  H. 
  Koppen 
  not 
  speaking 
  of 
  cineraseens, 
  

   it 
  would 
  be 
  interesting 
  to 
  know 
  whether 
  he 
  admits 
  this 
  species, 
  aud 
  if 
  

   in 
  the 
  affirmative, 
  whether 
  all 
  his 
  remarks 
  apply 
  alone 
  to 
  the 
  true 
  

   migraiorius 
  type, 
  notably 
  that 
  which 
  he 
  says 
  normally 
  sojourns 
  at 
  

   Bayoune, 
  where 
  I 
  have 
  taken 
  only 
  cineraseens, 
  variety 
  vireseens, 
  whose 
  

   characters 
  are 
  the 
  same 
  as 
  in 
  Belgium 
  and 
  Frankfort-on-the-Maiu. 
  It 
  

   is 
  also 
  cineraseens 
  that 
  M. 
  vou 
  Heyden 
  has 
  taken." 
  

  

  Some' 
  notes 
  on 
  the 
  Algerian 
  locusts 
  [Acrydium 
  peregrinum, 
  migrato- 
  

   rium, 
  &c.) 
  by 
  Ooure, 
  have 
  been 
  communicated 
  to 
  the 
  Entomological 
  

   Society 
  of 
  France 
  by 
  Giraud. 
  In 
  them, 
  mention 
  is 
  made 
  of 
  a 
  special 
  

   work 
  on 
  the 
  same 
  subject, 
  which 
  the 
  recorder 
  has 
  not 
  yet 
  seen. 
  (Bull. 
  

   Soc. 
  Ent. 
  Fr., 
  1867, 
  pp. 
  x, 
  xiii.) 
  The 
  locusts 
  visiting 
  Algeria 
  come 
  from 
  

   the 
  south, 
  aud 
  arrive 
  in 
  May. 
  The}^ 
  lay 
  their 
  eggs 
  soon 
  after 
  their 
  

   arrival, 
  and 
  the 
  young 
  animals 
  prodnced 
  from 
  these 
  eggs 
  usually 
  become 
  

   adult 
  in 
  July. 
  In 
  August 
  all 
  usually 
  disappear. 
  Coure 
  also 
  notices 
  the 
  

   arrival 
  in 
  Algeria 
  in 
  the 
  early 
  \rait 
  of 
  January, 
  18G7, 
  of 
  a 
  flight 
  of 
  

   locusts. 
  The 
  color 
  of 
  these 
  was 
  stated 
  to 
  be 
  reddish. 
  It 
  appears 
  that 
  

   on 
  (irst 
  attaining 
  their 
  adult 
  form, 
  these 
  insects 
  are 
  of 
  a 
  rosy 
  tint, 
  and 
  

   afterward 
  change; 
  and 
  Coure 
  thinks 
  that 
  it 
  is 
  not 
  until 
  after 
  their 
  

   change 
  of 
  color 
  that 
  they 
  are 
  fitted 
  for 
  reproduction. 
  Lallemant 
  states 
  

   (/. 
  c, 
  p. 
  xiii) 
  that 
  the 
  locusts, 
  which 
  live 
  for 
  a 
  long 
  time 
  in 
  the 
  adult 
  

  

  