756  Chemical  Elements  of  Living  Matter.  |  AmNJ00vur'i92iarm" 
Many  of  these  elments  are  associated  and  occur  in  pairs,  thus  in 
the  vegetable  metabolism  Mg  and  K  are  predominant,  while  the 
corresponding  pair  in  the  animal  organism  is  Fe  and  Na.  These 
two  pairs  are  also  found  in  igneous  rocks.8  Again  there  is  an  accum- 
ulation of  certain  elements  in  definite  tissues  of  the  organism,  thus 
in  the  vegetable  organism  the  seeds  are  rich  in  P  and  Mg  and  de- 
pleted of  Ca  and  K,  while  the  stems  and  leaves  contain  much 
Ca,  K,  and  often  Si,  and  the  roots  are  rich  in  K  and  depleted  of 
P. 
The  specific  role  of  the  elements  is  little  understood  and  so  it  is 
speculative  to  account  for  their  presence.  It  seems  that  the  presence 
of  an  element,  other  than  the  9  or  12  essential  ones,  is  the  result 
of  either  evolution  or  adaptation.  Thus  the  occurrence  of  iodine 
in  the  thyroid  gland  of  vertebrates  seems  the  result  of  evolution; 
e.  g.,  the  utilization  in  a  highly  specialized  organ  of  the  organism, 
while  its  presence  in  marine  plants  and  invertebrates  seems  the  result 
of  adaptation,  e.  g.,  diffused  throughout  the  cell  organism  with  ap- 
parently no  distinct  function.  Likewise  the  presence  of  copper  in 
the  blood  of,  certain  invertebrates  seems  due  to  evolution,  while 
its  presence  in  bacteria,  grown  successively  for  several  generations 
upon  a  copper  containing  culture  media,  is  adaptation. 
An  interesting  comparison  of  the  distribution  of  elements  in 
living  organisms  and  inanimate  nature  with  its  relation  to  the  per- 
iodic system  has  been  pointed  out  in  previous  papers7  where  it  was 
shown  that  the  elements  of  low  atomic  weights  essential  and  occur- 
ing  in  living  matter,  cluster  together  in  the  periodic  table  and  seem 
to  point  to  an  evolution.  There  are  many  unsolved  questions  in  this 
fascinating  problem.  Why  did  the  cell  substitute  in  specialized  tis- 
sues the  elements  of  higher  atomic  weight  for  those  of  lower  atomic 
weight,  e.  g.,  Br  and  I  for  CI  and  F?  Is  the  answer  merely  to  be 
found  in  physico-chemical  phenomena,  or  has  that  mysterious  con- 
trolling force  guarding  the  entrance  to  the  protoplasm  exerted  a 
selective  process  for  a  certain  end?  Is  this  force,  the  vital  force, 
possessed  of  intelligence  and  not  of  a  purely  physical  or  chemical 
nature  ? 
6  H.  S.  Washington,  Set.  Am.  Suppl.,  Vol.  83,  page  27,  1917. 
T  Journal  of  General  Physiology,  Vol.   1,  page  429,  1919,  and  Science 
Progress,  Vol.  14,  page  602,  1920. 
