﻿182 
  PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM 
  vou 
  i03 
  

  

  1945). 
  We 
  are 
  thus 
  justified 
  in 
  assuming 
  that 
  the 
  fresh-water 
  life 
  

   of 
  Alaska 
  was 
  not 
  completely 
  destroyed 
  even 
  at 
  the 
  peaks 
  of 
  glacia- 
  

   tion 
  when 
  practically 
  all 
  Canada 
  and 
  a 
  considerable 
  part 
  of 
  the 
  

   United 
  States 
  were 
  covered 
  with 
  ice 
  caps. 
  

  

  The 
  Alaskan 
  fresh-water 
  triclads 
  show 
  no 
  close 
  relationships 
  with 
  

   the 
  present 
  North 
  American 
  triclad 
  fauna 
  inhabiting 
  the 
  midwestem 
  

   plains 
  and 
  the 
  eastern 
  and 
  southern 
  areas 
  of 
  the 
  continent. 
  A 
  

   comparison 
  with 
  the 
  fauna 
  of 
  the 
  Rocky 
  Mountains 
  is 
  not 
  possible 
  

   at 
  present, 
  since 
  the 
  West 
  of 
  the 
  United 
  States 
  and 
  of 
  Canada 
  is 
  

   almost 
  unexplored 
  with 
  regard 
  to 
  triclads 
  and 
  to 
  lower 
  aquatic 
  in- 
  

   vertebrates 
  in 
  general. 
  In 
  any 
  event 
  it 
  appears 
  highly 
  improbable 
  

   that 
  Alaska 
  was 
  populated 
  by 
  species 
  migrating 
  to 
  it 
  from 
  the 
  south 
  

   or 
  east. 
  

  

  On 
  the 
  other 
  hand, 
  the 
  relations 
  of 
  the 
  Alaska 
  triclads 
  to 
  the 
  

   fauna 
  of 
  Eurasia 
  are 
  unmistakable. 
  Though, 
  according 
  to 
  our 
  

   present 
  knowledge, 
  none 
  of 
  the 
  Alaskan 
  triclads 
  is 
  specifically 
  identi- 
  

   cal 
  with 
  any 
  Asiatic 
  form, 
  two 
  species, 
  Phagocata 
  nivea 
  and 
  Polycelis 
  

   horealis, 
  are 
  very 
  closely 
  related 
  to 
  Asiatic 
  species 
  of 
  the 
  same 
  genera. 
  

  

  Figure 
  25 
  shows 
  the 
  geographic 
  range 
  of 
  the 
  genus 
  Polycelis 
  as 
  it 
  

   is 
  known 
  at 
  present. 
  The 
  individual 
  dots 
  on 
  the 
  map 
  represent 
  

   either 
  single 
  records 
  or 
  groups 
  of 
  neighboring 
  localities 
  where 
  species 
  

   of 
  the 
  genus 
  have 
  been 
  found. 
  In 
  interpreting 
  the 
  map 
  it 
  is 
  to 
  be 
  

   kept 
  in 
  mind 
  that 
  not 
  all 
  geographical 
  areas 
  are 
  equally 
  well 
  mvesti- 
  

   gated 
  with 
  regard 
  to 
  the 
  occurrence 
  of 
  lower 
  invertebrates. 
  Europe, 
  

   Japan, 
  and 
  the 
  eastern 
  parts 
  of 
  the 
  United 
  States 
  are 
  comparatively 
  

   well 
  known 
  while 
  our 
  knowledge 
  of 
  the 
  Asiatic, 
  Western 
  American, 
  

   and 
  African 
  triclads 
  is 
  still 
  rather 
  deficient. 
  A 
  study 
  of 
  the 
  map 
  

   suggests 
  that 
  Polycelis 
  is 
  primarily 
  a 
  Eurasian 
  genus. 
  It 
  appears 
  

   to 
  have 
  extended 
  its 
  range, 
  in 
  some 
  earlier 
  geological 
  period, 
  to 
  the 
  

   northern 
  rim 
  of 
  Africa.* 
  In 
  a 
  similar 
  way 
  it 
  may 
  have 
  migrated 
  into 
  

   Alaska 
  at 
  the 
  time 
  of 
  the 
  cenozoic 
  land 
  bridges 
  and 
  may 
  have 
  pene- 
  

   trated 
  south 
  along 
  the 
  Rocky 
  Mountains. 
  It 
  is 
  highly 
  probable 
  

   that 
  Polycelis 
  will 
  be 
  found, 
  in 
  the 
  future, 
  more 
  widely 
  distributed 
  

   in 
  the 
  Rockies 
  than 
  present 
  collection 
  records 
  indicate. 
  

  

  The 
  repeated 
  emergence 
  of 
  a 
  land 
  bridge 
  between 
  Alaska 
  and 
  the 
  

   eastern 
  tip 
  of 
  Siberia 
  is 
  generally 
  accepted 
  by 
  geologists 
  and 
  paleon- 
  

   tologists. 
  There 
  is 
  ample 
  evidence 
  of 
  an 
  exchange 
  of 
  faunal 
  elements 
  

   between 
  the 
  two 
  continents 
  (cf. 
  Simpson, 
  1940 
  and 
  1947). 
  The 
  most 
  

   recent 
  corridor 
  must 
  have 
  existed 
  during 
  the 
  glacial 
  stages 
  of 
  the 
  

   Pleistocene. 
  The 
  volume 
  of 
  ice 
  masses 
  accumulated 
  over 
  vast 
  areas 
  

   of 
  the 
  northern 
  hemisphere 
  has 
  been 
  estimated 
  conservatively 
  at 
  

   34 
  to 
  42 
  million 
  cubic 
  kilometers. 
  The 
  corresponding 
  depletion 
  of 
  

  

  ♦ 
  The 
  presence 
  of 
  Polycelis 
  oculi-marginata 
  (Palombi) 
  in 
  New 
  Guinea 
  is 
  a 
  zoogeographic 
  enigma 
  (cf. 
  Beau- 
  

   champ, 
  1947). 
  

  

  