From  the  foregoing  it  may  safely  be  inferred  that  as  the  deposit  was
built  up,  there  would  be  a  change  in  the  character  of  the  vegetation
as  the  water  decreased  in  depth  and  that  the  plants  concerned  in  the
formation  of  the  deposit  would  occupy  such  places  as  were  suited  to
their  requirements  in  a  definite  order.  The  same  conclusion  would  be
arrived  at  by  studying  the  plant  growth  from  the  margin  to  the  center

'  Pond,  R.  H.,  Report  U.  S.  Fish  Commission,    1903.
DAVIS    ON    PEAT.  133

of  any  shallow  lake  which  is  in  the  process  of  filling,  for  it  would  be
found  that  the  plants  definitely  arrange  themselves  according  to  the
depth  of  the  water,  and  if  the  slope  of  the  bottom  is  uniform  and  the
character  of  the  bottdjn  practically  the  same  throughout,  the  plants
of  given  types  usually  grow  in  well  marked  bands  or  zones,  entirely
around  the  shores.  The  descriptions  of  such  lakes  are  numerous  in  bo-
tanical literature  both  European  and  American  and  the  zonal  arrange-
ment of  the  plants  in  these  locations  a  well  recognized  ecological  phe-
nomenon.
In  studying  a  series  of  such  lakes  the  conclusion  must  be  reached
that  as  fast  as  conditions  are  favorable,  the  vegetation  of  the  shoreward
zones,  i.  e.  that  growing  in  shallower  water,  moves  toward  the  center,
and  is  replaced  by  that  from  nearer  the  shore.  This  is  due  not  only  to  the
shoaling  of  the  water  and  the  increased  light  which  reaches  the  plants
growing  upon  the  bottom,  but  also  to  increased  heat,  since  the  shallower
water  is  more  quickly  warmed  than  the  deeper,  to  improved  aeration  due
to  more  frequent  stirrings  by  wave  and  surface  current-action,  and
to  the  improved  physical  and  chemical  condition  of  the  substratum.
