On  the  other  hand,  there  are  obvious  difficulties  to  be
encountered  in  the  application  of  such  a  criterion.  As
regards  behavior,  the  world  of  Hfe  is  not  divided  into
two  distinct  groups  marked  by  specifically  different  reac-
tions, the  cephalic  and  the  non-cephalic.  The  develop-
ment of  behavior  presents  no  evident  breaks.  Higher
and  lower  groups  are  connected  by  a  series  of  inter-
mediate types  which  present  relative  continuity  in  their
relations.  Instead  of  being  far  withdrawn  from  each
other,  the  one  group  passes  into  the  other  by  insensible
gradations.  If  there  be  grounds,  as  regards  external
organization,  for  the  limitation  of  the  field  of  conscious-
ness, it  cannot  be  said  to  have  its  justification  in  any
sharp  delimitation  of  two  classes  of  behavior  associated
with  these  organic  types  respectively.
Further,  if,  regarding  behavior  as  the  objective  mani-
festation of  organization,  we  turn  to  the  question  of
structure,  the  same  lack  of  demarcation  will  be  found
there  also.  The  world  of  life  is  not  divided  into  two
isolated  and  contrasted  groups,  one  possessing  a  brain
and  the  other  lacking  it.  The  cephalic  type  presents  a
range  of  variations  marked  by  very  great  differences  in
complexity  and  integrity  of  organization  between  its
highest  and  lowest  members.  The  non-cephalic  group,
likewise,  includes  such  extremes  as  the  complete  absence
of  nervous  elements  in  the  protozoa  and  the  high  devel-
opment of  the  segmented  type  in  the  vermes.
Moreover,  the  two  groups  are  in  immediate  contact
at  their  edges.  No  anatomical  chasm  separates  the  two,
such  as  divides  any  two  species  separated  by  inter-
mediate forms.  When  followed  downward,  the  cephalic
type  is  found  to  disappear  in  forms  difficult  to  classify.
