﻿[31] 
  DEVELOPMENT 
  OF 
  THE 
  CETACEA. 
  457 
  

  

  pertropliied, 
  obviously 
  in 
  correlation 
  with 
  the 
  vast 
  supra 
  and 
  infra 
  

   caudal 
  musculature, 
  while 
  the 
  second 
  group 
  shows 
  unmistakable 
  signs 
  

   of 
  gradual 
  degeneracy 
  increasing 
  from 
  the 
  first 
  to 
  the 
  last. 
  The 
  change 
  

   from 
  the 
  type 
  of 
  the 
  first 
  series 
  to 
  that 
  of 
  the 
  posterior 
  or 
  second 
  one 
  is 
  

   remarkably 
  abrupt 
  in 
  some 
  forms, 
  the 
  latter 
  exhibiting 
  degeneracy 
  in 
  

   the 
  most 
  striking 
  way 
  by 
  the 
  loss 
  of 
  the 
  cylindrical 
  form 
  characteristic 
  

   of 
  the 
  centra 
  of 
  the 
  first 
  part 
  of 
  the 
  caudal 
  series 
  and 
  the 
  assumption 
  

   of 
  a 
  depressed, 
  rounded, 
  or 
  in 
  some 
  cases 
  almost 
  globular 
  form. 
  

  

  This 
  uniformity 
  in 
  the 
  length 
  of 
  the 
  centra 
  of 
  the 
  first 
  subdivision 
  of 
  

   the 
  caudal 
  bones 
  is 
  probably 
  an 
  adaptive 
  character, 
  and 
  one 
  which 
  has 
  

   been 
  evolved 
  pari 
  passu 
  with 
  the 
  differentiation 
  of 
  the 
  great 
  caudal 
  mus- 
  

   cles, 
  the 
  caudal 
  skeleton 
  and 
  musculature 
  actually 
  assuming 
  the 
  func- 
  

   tion 
  of 
  the 
  hind 
  limbs. 
  The 
  hypertrophy, 
  however, 
  which 
  we 
  have 
  

   noticed 
  must, 
  since 
  it 
  would 
  have 
  tended 
  to 
  increase 
  the 
  length 
  of 
  each 
  

   one 
  of 
  the 
  first 
  series 
  of 
  centra, 
  also 
  have 
  tended 
  to 
  lengthen, 
  as 
  well 
  as 
  

   strengthen, 
  the 
  tail, 
  and 
  thus 
  aid 
  in 
  carrying 
  the 
  flukes 
  farther 
  back 
  

   from 
  their 
  original 
  position 
  in 
  the 
  ancestral 
  cetacean 
  type, 
  with 
  the 
  

   thighs 
  and 
  legs 
  of 
  its 
  hind 
  limbs 
  bound 
  together 
  with 
  the 
  tail 
  by 
  in 
  

   tecfument, 
  and 
  when 
  the 
  tail 
  must 
  still 
  have 
  been 
  extended 
  between 
  

   the 
  pedes, 
  this 
  supposed 
  ancestral 
  form 
  doubtless 
  being 
  the 
  possessor 
  

   of 
  a 
  greater 
  number 
  of 
  caudal 
  vertebrae 
  than 
  are 
  found 
  in 
  the 
  existing 
  

   seals. 
  

  

  We 
  at 
  any 
  rate 
  find 
  nothing 
  like 
  such 
  a 
  remarkable 
  differentiation 
  of 
  

   the 
  caudal 
  vertebrae 
  in 
  any 
  other 
  mammals 
  except 
  cetaceans, 
  and 
  the 
  

   inference 
  is 
  that 
  such 
  a 
  differentiation 
  into 
  regions 
  is 
  intimately 
  bound 
  

   up 
  with 
  the 
  acquirement 
  of 
  an 
  important 
  new 
  function 
  in 
  connection 
  

   with 
  the 
  outward 
  vestiges 
  of 
  the 
  feet 
  now 
  borne 
  upon 
  its 
  sides, 
  and 
  

   which, 
  by 
  the 
  process 
  of 
  hypertrophy 
  of 
  the 
  anterior 
  series 
  of 
  osseous 
  

   caudal 
  segments, 
  it 
  aided 
  in 
  still 
  farther 
  translocating 
  backward 
  after 
  

   the 
  skeleton 
  and 
  muscles 
  of 
  the 
  hind 
  limb 
  had 
  atrophied 
  in 
  the 
  ances- 
  

   tral 
  type. 
  In 
  other 
  mammals 
  the 
  centra 
  of 
  the 
  caudal 
  vertebrae 
  at 
  once 
  

   gradually 
  diminish 
  in 
  vertical 
  diameter 
  from 
  the 
  sacrum 
  backwards 
  to 
  

   the 
  end 
  of 
  the 
  tail, 
  the 
  posterior 
  ones 
  often 
  tending 
  to 
  become 
  depressed, 
  

   a 
  tendency 
  which 
  is 
  also 
  exhibited 
  by 
  the 
  posterior 
  or 
  second 
  caudal 
  

   series 
  of 
  whales. 
  This 
  character 
  places 
  the 
  cetaceans 
  in 
  contrast 
  with 
  

   all 
  other 
  mammals 
  except, 
  probably, 
  Halitherium, 
  Ehytina, 
  and 
  Halicore. 
  

  

  " 
  Muscles 
  which 
  represent 
  the 
  psoas 
  minor 
  and 
  the 
  subvertebral 
  mus- 
  

   cles 
  of 
  the 
  Cetacea 
  are 
  very 
  large 
  and 
  play 
  an 
  important 
  part 
  in 
  effect- 
  

   ing 
  the 
  locomotion 
  of 
  the 
  seal," 
  says 
  Huxley, 
  so 
  that 
  we 
  actually 
  find 
  

   that 
  a 
  beginning 
  has 
  been 
  made 
  in 
  the 
  development 
  of 
  an 
  axial 
  mus- 
  

   cular 
  apparatus 
  in 
  the 
  seal 
  which 
  in 
  the 
  Cetacea 
  has 
  been 
  extended 
  both 
  

   forward 
  and 
  backward 
  and 
  has 
  attained 
  tremendous 
  proportions. 
  The 
  

   movements 
  of 
  pinnipeds 
  and 
  cetaceans 
  in 
  the 
  water 
  are 
  somewhat 
  sim- 
  

   ilar. 
  Both 
  can 
  in 
  fact 
  move 
  rapidly 
  along 
  an 
  undulating 
  course 
  by 
  flexing 
  

   the 
  hinder 
  part 
  of 
  the 
  body 
  up 
  and 
  down. 
  Such 
  a 
  similarity 
  in 
  the 
  

   habits 
  of 
  movement 
  of 
  the 
  two 
  animals 
  it 
  is 
  hard 
  to 
  believe 
  are 
  not 
  re- 
  

  

  