﻿Wieland 
  — 
  Evolution 
  of 
  the 
  Testudinate 
  Humerus. 
  413 
  

  

  Aet. 
  XLII. 
  — 
  Some 
  Observations 
  on 
  Certain 
  Well-Marked 
  

   Stages 
  in 
  the 
  Evolution 
  of 
  the 
  Testudinate 
  Humerus 
  ; 
  by 
  

   G. 
  R. 
  Wieland. 
  

  

  The 
  study 
  of 
  the 
  morphology 
  and 
  development 
  of 
  an 
  

   important 
  skeletal 
  part 
  throughout 
  an 
  order 
  must, 
  as 
  knowl- 
  

   edge 
  increases, 
  throw 
  much 
  light 
  on 
  the 
  evolution 
  of 
  the 
  order 
  

   itself. 
  How 
  far 
  in 
  the 
  definite 
  case 
  here 
  presented, 
  the 
  Testu- 
  

   dinate 
  humerus 
  reflects 
  the 
  evolution 
  of 
  the 
  Testudinata 
  is 
  a 
  

   difficult 
  question 
  to 
  answer 
  even 
  in 
  general 
  terms 
  and 
  as 
  yet 
  

   impossible 
  to 
  deal 
  with 
  finally. 
  Nevertheless 
  its 
  present 
  con- 
  

   sideration, 
  as 
  based 
  upon 
  the 
  fundamental 
  principles 
  of 
  evolu- 
  

   tion, 
  makes 
  possible 
  a 
  clearer 
  discernment 
  of 
  some 
  valuable 
  

   data. 
  

  

  Obviously 
  if 
  we 
  select 
  a 
  specialized 
  organ 
  in 
  a 
  specialized 
  

   group 
  we 
  may 
  expect 
  that 
  the 
  factors 
  which 
  have 
  been 
  potent 
  

   in 
  moulding 
  it 
  will 
  have 
  been 
  on 
  the 
  one 
  hand 
  restricted 
  in 
  

   their 
  action 
  and 
  on 
  the 
  other 
  more 
  strongly 
  thrown 
  into 
  relief. 
  

   Such 
  an 
  example 
  is 
  the 
  Testudinate 
  humerus, 
  presenting 
  also 
  

   many 
  advantages 
  of 
  study. 
  For 
  there 
  is 
  in 
  the 
  turtles 
  to 
  a 
  

   degree 
  scarcely 
  approached 
  in 
  any 
  other 
  order 
  a 
  graduated 
  

   change 
  in 
  habitat 
  from 
  dry 
  deserts 
  to 
  the 
  ocean, 
  with 
  varied 
  

   feeding 
  habits, 
  as 
  well 
  as 
  a 
  wide 
  distribution 
  of 
  both 
  living 
  

   and 
  extinct 
  forms 
  in 
  latitude 
  and 
  time. 
  

  

  The 
  two 
  great 
  groups 
  of 
  existing 
  Testudinates, 
  the 
  land 
  and 
  

   the 
  marine, 
  the 
  clawed 
  and 
  the 
  flippered 
  forms, 
  each 
  possess 
  a 
  

   characteristic 
  humerus 
  which 
  can 
  never 
  be 
  mistaken 
  the 
  one 
  

   for 
  the 
  other. 
  But 
  if 
  various 
  intervening 
  fossil 
  types 
  chiefly 
  

   of 
  Mesozoic 
  age 
  be 
  considered, 
  it 
  at 
  once 
  becomes 
  evident 
  that 
  

   there 
  has 
  been 
  a 
  steady 
  and 
  well-illustrated, 
  transition 
  from 
  

   whatever 
  was 
  the 
  original 
  form 
  toward 
  the 
  humerus 
  seen 
  in 
  

   the 
  most 
  highly 
  specialized 
  land 
  turtles 
  in 
  the 
  one 
  direction, 
  

   and 
  toward 
  that 
  of 
  the 
  most 
  highly 
  specialized 
  or 
  older 
  sea 
  

   turtles 
  in 
  the 
  other. 
  That 
  is, 
  a 
  series 
  may 
  be 
  discerned 
  with 
  

   well-marked 
  stages 
  easily 
  graduating 
  into 
  each 
  other, 
  at 
  the 
  

   one 
  end 
  of 
  which 
  stands 
  the 
  humeral 
  form 
  seen 
  in 
  various 
  

   genera 
  of 
  the 
  Testudinidw, 
  and 
  at 
  the 
  other 
  that 
  of 
  Dennochelys, 
  

   which 
  is 
  either 
  a 
  very 
  ancient, 
  or 
  else 
  the 
  most 
  highly 
  special- 
  

   ized 
  marine 
  turtle 
  known. 
  

  

  Of 
  these 
  stages 
  the 
  attempt 
  is 
  here 
  made 
  to 
  point 
  out 
  and 
  

   describe 
  six. 
  In 
  doing 
  so 
  it 
  should 
  perhaps 
  be 
  stated 
  at 
  the 
  

   outset 
  that 
  while 
  variations 
  in 
  humeral 
  outline 
  can 
  scarcely 
  be 
  

   conceived 
  of 
  except 
  as 
  being 
  chiefly 
  adaptive 
  in 
  character, 
  

   being 
  so 
  intimately 
  connected 
  with 
  habitat, 
  it 
  will 
  be 
  seen 
  that 
  

  

  Am. 
  Jour. 
  Sci. 
  — 
  Fourth 
  Series, 
  Yol. 
  IX, 
  No. 
  54. 
  — 
  June, 
  1900. 
  

   29 
  

  

  