The  period  in  which  we  live  is,  as  regards  our
northern  hemisphere,  half-way  between  these  ex-
tremes, or  temperate  in  its  climate.  We  are  passing
from  the  last  extreme  condition  of  Arctic  cold  (during
the  Glacial  Period),  slowly,  to  an  infinitesimally
increasing  state  of  warmth,  which  may  culminate
ages  hence,  in  a  tropical  warmth  similar  to  that
which  geologists  are  perfectly  assured  existed  in  this
country  during  the  Eocene  Period  of  Tertiary  time,
when  the  familiar  blue  clay  cut  into  during  the


286          SAGACITY  AND  MOKALITY  OF  PLANTS.
too    frequent    metropolitan    excavations  was    being
slowly  accumulated  on  a  shallow  sea-floor.
For  ages  before  that — indeed  ever  since  the  dry
land  received  its  first  lowly -organised  vegetation  in
Cambrian  times — this  slow  beat  of  climatic  changes
has  repeatedly  taken  place.  What  strange  extremes
the  land -plants  must  have  been  subjected-  to  !  It
is  as  if  the  Esquimaux  had  been  repeatedly  trans-
ported to  Calcutta,  and  the  Hindoos  forced  to  take
up  their  abode  amidst  the  eternal  frosts  and  snows
of  the  Arctic  regions  !

It  must  be  further  remembered  that  these
climatic  variations  have  been  accompanied  by
equally  slow  but  certain  geographical  changes,
which  have  converted  dry  land  into  sea -bed,  and
upheaved  the  bottom  of  the  sea  to  form  new  dry
lands — which  have  crumpled  up  previously  hori-
zontal strata  into  tablelands  and  mountain -chains  ;
or  depressed  those  already  existing  until  they  were
let  down  in  the  middle  like  a  bellying  chain,  as  is
the  case  with  the  underground  connection  between
the  Mendip  and  Ardennes  Hills,  beneath  London.
