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  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1922. 
  

  

  orthogenetic 
  phenomena 
  as 
  have 
  been 
  observed 
  are 
  better 
  interpreted 
  

   as 
  analogous 
  to 
  chemical 
  phenomena, 
  where 
  tendency 
  to 
  certain 
  re- 
  

   actions 
  is 
  greater 
  than 
  to 
  others, 
  than 
  as 
  " 
  vital 
  force 
  " 
  phenomena. 
  

   No 
  one 
  can 
  maintain 
  that 
  these 
  genetic 
  findings 
  compare 
  with 
  the 
  

   fundamental 
  laws 
  of 
  thermodynamics 
  in 
  elegance 
  and 
  simplicity. 
  

   No 
  one 
  can 
  say 
  how 
  general 
  they 
  are. 
  But 
  fruit 
  flies 
  and 
  maize, 
  

   rodents 
  and 
  peas, 
  upon 
  which 
  the 
  observations 
  were 
  largely 
  made, 
  

   are 
  pretty 
  far 
  apart 
  in 
  the 
  general 
  scheme 
  of 
  things; 
  therefore 
  it 
  

   would 
  be 
  very 
  odd 
  indeed 
  if 
  they 
  should 
  turn 
  out 
  to 
  be 
  special 
  cases. 
  

   And 
  to 
  me 
  they 
  are 
  very 
  helpful 
  to 
  a 
  clearer 
  general 
  conception 
  of 
  

   evolution. 
  

  

  THE 
  MECHANISM 
  OF 
  HEREDITY. 
  

  

  Turning 
  now 
  to 
  the 
  mechanism 
  of 
  heredity, 
  let 
  us 
  see 
  what 
  can 
  be 
  

   said. 
  The 
  main 
  generalization 
  is 
  that 
  there 
  are 
  units 
  of 
  inheritance, 
  

   the 
  genes, 
  which 
  are 
  constant 
  in 
  the 
  sense 
  that 
  stable 
  chemical 
  com- 
  

   pounds 
  are 
  constant 
  ; 
  and 
  whose 
  distribution 
  follows 
  the 
  distribution 
  

   of 
  the 
  chromosomes. 
  In 
  other 
  words, 
  the 
  discoveries 
  of 
  experimental 
  

   genetics 
  have 
  made 
  it 
  possible 
  to 
  endow 
  the 
  conceptual 
  units 
  of 
  

   earlier 
  days 
  with 
  particular 
  qualities, 
  just 
  as 
  discoveries 
  in 
  the 
  

   physical 
  sciences 
  have 
  made 
  it 
  possible 
  to 
  delimit 
  the 
  characteristics 
  

   of 
  atoms 
  and 
  molecules. 
  Presumably 
  there 
  may 
  be 
  other 
  types 
  of 
  

   inheritance, 
  but 
  the 
  only 
  one 
  thus 
  far 
  described 
  is 
  an 
  exclusively 
  

   maternal 
  inheritance 
  of 
  certain 
  plastid 
  characters. 
  And 
  even 
  in 
  

   this 
  case, 
  it 
  is 
  not 
  absolutely 
  certain 
  we 
  are 
  not 
  dealing 
  with 
  sym- 
  

   biotic 
  organisms 
  that 
  are 
  transferred 
  from 
  host 
  to 
  host 
  in 
  some 
  such 
  

   way 
  as 
  the 
  numerous 
  more 
  or 
  less 
  yeast-like 
  forms 
  being 
  daily 
  de- 
  

   scribed 
  in 
  relation 
  to 
  insects 
  and 
  other 
  lower 
  animals. 
  The 
  mere 
  

   fact 
  that 
  numerous 
  dicotyledons 
  and 
  monocotyledons 
  on 
  the 
  one 
  

   hand, 
  and 
  mammals, 
  birds, 
  amphibians, 
  reptiles, 
  fishes, 
  arthropods, 
  

   and 
  molluscs 
  on 
  the 
  other, 
  show 
  essentially 
  identical 
  types 
  of 
  here- 
  

   dity, 
  makes 
  it 
  probable 
  that 
  the 
  generalized 
  mechanism 
  has 
  been 
  

   discovered. 
  At 
  the 
  same 
  time, 
  though 
  the 
  angiosperms, 
  insects, 
  and 
  

   mammals 
  thus 
  far 
  studied 
  intensively, 
  distribute 
  their 
  units 
  of 
  here- 
  

   dity 
  with 
  a 
  convincing 
  similarity 
  of 
  detail, 
  it 
  is 
  altogether 
  likely 
  that 
  

   special 
  cases 
  of 
  peculiar 
  distributions 
  will 
  be 
  found 
  later. 
  And 
  there 
  

   is 
  every 
  reason 
  to 
  believe 
  that 
  these 
  odd 
  or 
  unique 
  types 
  of 
  inheritance 
  

   will 
  parallel 
  a 
  specialization 
  in 
  chromosome 
  distribution 
  departing 
  

   somewhat 
  from 
  the 
  one 
  we 
  have 
  come 
  to 
  look 
  upon 
  as 
  regular. 
  

  

  THE 
  LAWS 
  OF 
  HEREDITY. 
  

  

  This 
  regular 
  or 
  common 
  scheme 
  of 
  chromosome 
  distribution 
  in 
  

   sexual 
  reproduction 
  has 
  been 
  the 
  basis 
  of 
  practically 
  all 
  of 
  our 
  pres- 
  

   ent 
  genetic 
  knowledge. 
  From 
  past 
  experience 
  with 
  it 
  one 
  can 
  pick 
  

   out 
  the 
  following 
  inductions, 
  each 
  of 
  which 
  has 
  been 
  tested 
  with 
  

  

  