﻿454 
  W. 
  F. 
  FISKE. 
  

  

  Apart 
  from 
  these 
  things 
  the 
  insect 
  appears 
  to 
  require 
  nothing 
  except 
  that 
  which 
  

   is 
  inseparable 
  from 
  an 
  equable, 
  tropical 
  climate. 
  It 
  is 
  believed 
  to 
  be 
  pure 
  coincidence 
  

   that 
  the 
  insect 
  is 
  never 
  found 
  far 
  from 
  water, 
  and 
  it 
  is 
  confidently 
  believed 
  that 
  it 
  

   would 
  exist, 
  and 
  that 
  it 
  will 
  be 
  found 
  eventually 
  to 
  exist, 
  in 
  any 
  inland 
  localities 
  

   where 
  host 
  animals 
  of 
  favoured 
  species 
  occur 
  in 
  well 
  sheltered 
  areas 
  provided 
  

   with 
  suitable 
  breeding 
  places. 
  In 
  fact 
  the 
  interior 
  infestation 
  on 
  Mbugwe 
  Island, 
  

   described 
  on 
  page 
  384 
  and 
  mentioned 
  in 
  several 
  other 
  connections, 
  is 
  believed 
  to 
  be 
  

   a 
  true 
  inland 
  colony, 
  such 
  as 
  might 
  occur 
  equally 
  well 
  at 
  any 
  distance 
  from 
  lake 
  

   or 
  river 
  under 
  a 
  comparably 
  favourable 
  (and 
  extremely 
  unusual) 
  combination 
  of 
  

   circumstances. 
  

  

  The 
  three 
  requisites 
  being 
  equally 
  indispensable, 
  it 
  follows 
  that 
  in 
  the 
  absence 
  

   of 
  any 
  one 
  the 
  species 
  cannot 
  exist. 
  All 
  three 
  must 
  be 
  present 
  in 
  proper 
  combina- 
  

   tion, 
  and 
  local 
  density 
  is 
  likely 
  to 
  be 
  governed 
  by 
  the 
  quantity 
  or 
  amount 
  of 
  

   whichever 
  one 
  of 
  the 
  three 
  is 
  least 
  abundantly 
  provided 
  : 
  — 
  

  

  (a) 
  by 
  amount 
  of 
  food, 
  if 
  protection 
  is 
  more 
  than 
  sufficient, 
  relatively 
  ; 
  (6) 
  by 
  

   amount 
  of 
  shelter, 
  if 
  food 
  and 
  breeding 
  ground 
  are 
  both 
  more 
  than 
  sufficient, 
  

   relatively 
  ; 
  and 
  (c) 
  by 
  amount 
  of 
  breeding 
  ground, 
  if 
  food 
  and 
  shelter 
  are 
  both 
  

   more 
  than 
  sufficient, 
  relatively. 
  

  

  This 
  is 
  indeed 
  the 
  case. 
  In 
  the 
  Bwendi 
  district 
  (Sect. 
  VI 
  (g) 
  ) 
  shelter 
  and 
  breeding 
  

   grounds 
  were 
  both 
  very 
  good 
  and 
  extensive, 
  but 
  food 
  was 
  very 
  scanty 
  and 
  density 
  of 
  

   fly 
  low; 
  along 
  the 
  southern 
  shore 
  of 
  Buganga 
  Peninsula, 
  (p. 
  432, 
  Table 
  XXXIV) 
  

   food 
  and 
  breeding 
  grounds 
  were 
  both 
  good 
  and 
  plentiful, 
  but 
  shelter 
  was 
  poor 
  or 
  

   lacking, 
  and 
  density 
  of 
  fly 
  was 
  therefore 
  low 
  ; 
  on 
  Bugaba 
  Island 
  (p. 
  424) 
  both 
  

   food 
  and 
  shelter 
  were 
  good 
  and 
  plentiful, 
  but 
  breeding 
  grounds 
  were 
  poor, 
  or 
  few, 
  

   and 
  density 
  of 
  fly 
  was 
  low. 
  These 
  are 
  merely 
  specific 
  instances 
  selected 
  to 
  illustrate 
  

   the 
  general 
  rule, 
  which 
  was 
  upheld 
  everywhere. 
  

  

  A 
  very 
  important 
  question 
  is 
  : 
  What 
  constitutes 
  a 
  proper 
  combination 
  of 
  the 
  

   requisites 
  to 
  life 
  ? 
  It 
  is 
  obvious 
  that 
  they 
  must 
  occur 
  within 
  the 
  radius 
  of 
  movement 
  

   and 
  perception 
  of 
  individual 
  insects, 
  from 
  a 
  central 
  point, 
  or 
  from 
  each 
  other 
  ; 
  but 
  

   this 
  does 
  not 
  answer 
  the 
  question. 
  What 
  is 
  the 
  radius 
  of 
  movement 
  and 
  perception 
  — 
  

   or 
  it 
  may 
  be 
  called 
  the 
  radius 
  of 
  range 
  or 
  reach 
  — 
  of 
  individual 
  insects 
  ? 
  This 
  

   question 
  can 
  only 
  be 
  answered 
  empirically, 
  and 
  unfortunately 
  it 
  did 
  not 
  occur 
  

   to 
  me 
  until 
  nearly 
  the 
  close 
  of 
  the 
  investigations 
  to 
  attempt 
  to 
  answer 
  it. 
  Careful 
  

   review 
  of 
  field 
  notes, 
  supplemented 
  by 
  memory 
  of 
  conditions 
  not 
  accurately 
  described, 
  

   leads 
  to 
  the 
  tentative 
  conclusion, 
  that 
  unless 
  all 
  these 
  requisites 
  to 
  life 
  occur 
  within 
  

   a 
  radius 
  of 
  less 
  than 
  100 
  yards 
  from 
  a 
  central 
  point, 
  the 
  conditions 
  of 
  life 
  are 
  so 
  

   unfavourable 
  that 
  the 
  species 
  cannot 
  exist. 
  

  

  This 
  does 
  not 
  mean 
  that 
  territory 
  deficient 
  in 
  one 
  or 
  more 
  of 
  the 
  requisites 
  to 
  life 
  

   is 
  never 
  infested. 
  On 
  the 
  contrary, 
  in 
  the 
  major 
  portion 
  of 
  the 
  infested 
  territory 
  

   conditions 
  of 
  life 
  are 
  unfavourable 
  to 
  the 
  existence 
  of 
  the 
  species, 
  and 
  these 
  portions 
  

   owe 
  their 
  infestations 
  to 
  their 
  contiguity 
  to 
  territory 
  in 
  which 
  the 
  life 
  requisites 
  

   occur 
  in 
  proper 
  combination, 
  and 
  which 
  thus 
  becomes 
  a 
  centre 
  of 
  infestation 
  for 
  

   a 
  considerable 
  zone 
  surrounding. 
  Within 
  these 
  centres 
  natural 
  increase 
  of 
  fly 
  

   from 
  generation 
  to 
  generation 
  exceeds 
  mortality, 
  but 
  in 
  addition 
  to 
  loss 
  through 
  

   mortality 
  there 
  is 
  loss 
  through 
  emigration 
  or 
  dispersion 
  of 
  flies 
  into 
  the 
  surrounding 
  

  

  