﻿438 
  REPORT 
  OF 
  COMMISSIONER 
  OF 
  FISH 
  AND 
  FISHERIES. 
  [12 
  J 
  

  

  The 
  more 
  or 
  less 
  complete 
  abortion 
  of 
  the 
  interdigital 
  emarginations 
  

   in 
  the 
  maims 
  and 
  lies 
  of 
  pinnipeds 
  has 
  been 
  quite 
  completed 
  in 
  the 
  

   cetaceans 
  ; 
  the 
  digits 
  themselves 
  having 
  become 
  in 
  the 
  latter 
  much 
  pro- 
  

   longed 
  in 
  the 
  anterior 
  limbs 
  in 
  which 
  they 
  have 
  also 
  acquired 
  an 
  in- 
  

   creased 
  number 
  of 
  often 
  very 
  short 
  phalanges, 
  sometimes 
  as 
  many 
  as 
  

   nine 
  or 
  even 
  more 
  in 
  the 
  second 
  and 
  third 
  digits. 
  This 
  condition 
  is, 
  

   however, 
  probably 
  an 
  adaptive 
  structure, 
  evolved 
  in 
  consequence 
  of 
  the 
  

   acquisition 
  of 
  additional 
  segments 
  necessary 
  when 
  they 
  first 
  perma- 
  

   nently 
  took 
  to 
  the 
  median 
  in 
  which 
  they 
  now 
  live 
  and 
  through 
  whicli 
  

   they 
  move. 
  The 
  tendency 
  to 
  abbreviate 
  the 
  mobile 
  part 
  of 
  the 
  limbs 
  

   and 
  shorten 
  the 
  segments 
  is 
  manifest 
  in 
  the 
  construction 
  of 
  the 
  limbs 
  

   of 
  almost 
  all 
  the 
  fishes 
  of 
  the 
  group 
  Lyrifcra, 
  in 
  which 
  it 
  has 
  also 
  been 
  

   necessary, 
  as 
  in 
  the 
  whales 
  and 
  pinnipeds 
  to 
  shift, 
  the 
  insertions 
  of 
  the 
  

   flexor 
  and 
  extensor 
  muscles 
  outward 
  upon 
  the 
  more 
  distal 
  elements, 
  so 
  

   as 
  to 
  render 
  them 
  physiologically 
  more 
  effective. 
  In 
  most 
  of 
  the 
  Lyri- 
  

   /era 
  the 
  muscles 
  which 
  move 
  the 
  fins 
  have 
  the 
  most 
  peripheral 
  insertion 
  

   of 
  any 
  to 
  be 
  found 
  within 
  the 
  limits 
  of 
  the 
  Vertebrata, 
  or 
  upon 
  the 
  prox- 
  

   imal 
  ends 
  of 
  the 
  many-jointed 
  rays 
  shown 
  by 
  me 
  to 
  develop 
  beneath 
  the 
  

   epidermis 
  from 
  coalesced 
  rods 
  or 
  primitive 
  unjointed 
  rays, 
  just 
  as 
  the 
  

   limbs 
  of 
  the 
  higher 
  Vertebrata 
  at 
  first 
  develop 
  with 
  an 
  unjointed 
  bar 
  of 
  

   cartilage 
  extending 
  through 
  their 
  central 
  axes. 
  

  

  While, 
  traces 
  of 
  interdigital 
  emarginations 
  are 
  visible 
  in 
  the 
  fore 
  limb 
  

   of 
  cetacean 
  embryos 
  they 
  are 
  much 
  more 
  distinctly 
  visible 
  in 
  the 
  em- 
  

   bryos 
  of 
  pinnipeds 
  where 
  they 
  are 
  better 
  developed 
  than 
  in 
  the 
  adult 
  

   of 
  the 
  latter, 
  the 
  meaning 
  of 
  which 
  facts 
  will 
  be 
  clear 
  to 
  The 
  evolution- 
  

   ist.* 
  The 
  increase 
  in 
  the 
  number 
  of 
  phalangeal 
  segments, 
  however, 
  be- 
  

   gins 
  abruptly 
  in 
  the 
  cetaceans 
  ; 
  in 
  most 
  forms 
  the 
  middle 
  digits 
  contain 
  

   more 
  than 
  the 
  number 
  three, 
  normal 
  to 
  the 
  mammaliai#class, 
  so 
  that 
  the 
  

   whales 
  and 
  porpoises 
  are, 
  in 
  this 
  respect, 
  very 
  different 
  from 
  the 
  pinni- 
  

   peds. 
  But 
  it 
  must 
  be 
  borne 
  in 
  mind 
  that 
  there 
  are 
  at 
  least 
  two 
  prece- 
  

   dents 
  of 
  this 
  kind 
  which 
  occur 
  within 
  the 
  limits 
  of 
  the 
  Beptilia 
  which 
  

   have 
  apparently, 
  like 
  the 
  cetaceans, 
  been 
  modified 
  for 
  an 
  exclusively 
  

   marine 
  or 
  aquatic 
  existence; 
  these 
  are 
  the 
  Plesiosauria 
  and 
  Ichthyosau; 
  

   ria. 
  As 
  many 
  as 
  nine 
  phalanges 
  are 
  found 
  in 
  a 
  single 
  digit 
  of 
  the 
  pes 
  

   of 
  the 
  first 
  type, 
  and 
  twenty-six 
  in 
  the 
  third 
  digit 
  of 
  the 
  manus 
  of 
  Ichthy- 
  

   osaurus 
  intermedins. 
  These 
  forms 
  seem 
  iudeed 
  to 
  have 
  represented 
  the 
  

   existing 
  Cetaceans 
  in 
  the 
  seas 
  of 
  -the 
  Mesozoic 
  ages. 
  

  

  This 
  parallel 
  is 
  rendered 
  still 
  more 
  striking 
  when 
  we 
  recall 
  the 
  circum- 
  

   stance 
  that 
  in 
  Ichthyosaurus, 
  as 
  in 
  Cetaceans 
  and 
  Sirenians, 
  the 
  pelvis 
  

   is 
  not 
  connected 
  with 
  the 
  spinal 
  column, 
  and 
  that 
  it 
  has 
  evidently 
  under- 
  

   gone 
  considerable 
  degeneration, 
  but 
  in 
  Ichthyosaurus 
  there 
  has 
  been 
  

   no 
  shitting 
  of 
  the 
  distal 
  part 
  of 
  the 
  hind 
  limb 
  backwards, 
  as 
  in 
  Pinni- 
  

   peds 
  and 
  Cetaceans. 
  The 
  hind 
  limb 
  derived, 
  in 
  Ichthyosaurus, 
  from 
  that 
  

  

  * 
  Irregular 
  emarginations 
  arc 
  noticeable 
  on 
  the 
  hind 
  margin 
  of 
  the 
  flukes 
  of 
  Meg- 
  

   aptera. 
  According 
  to 
  Eschricht 
  such 
  irregular 
  emarginations 
  arc 
  visible 
  along- 
  the 
  

   posterior 
  border 
  of 
  the 
  flukes 
  of 
  an 
  advanced 
  foetus 
  of 
  a 
  species 
  of 
  this 
  genus. 
  

  

  