those  which  indirectly  favour  reproduction  and  which
might  conceivably  be  developed  by  sexual  selection  as
understood  by  Darwin,  whether  or  not  they  actually
are  so  developed.
There  are,  however,  other  sexual  differences  which
do  not  so  easily  fall  into  this  group.  These  differences
are  less  obvious;  many  of  them  are  relative,  or  only
perceptible  when  we  take  averages  into  consideration ;
but  they  are  very  numerous.  Thus  we  have,  for  in-
stance, the  much  greater  shallowness,  proportionately,
of  the  female  skull;  we  have  the  greater  size  and
activity  of  the  thyroid  gland  in  women  and  the
smaller  average  proportion  of  red  blood  corpuscles;
and  we  have  a  different  average  relationship  of  the
parts  of  the  brain  to  each  other.  These  differences
are  probably  related  indirectly  to  primary  and
secondary  sexual  differences;  they  are  not  of  great
importance  from  the  zoological  point  of  view,  but  they
are  of  considerable  interest  from  the  anthropological
point  of  view,  very  often  of  interest  from  the  patho-
logical point  of  view,  and  occasionally  of  great
interest  from  the  social  point  of  view.  They  cannot
be  easily  put  into  the  same  group  as  the  secondary
sexual  characters  as  usually  understood;  and  perhaps
it  would  be  convenient  if  we  were  to  agree  to  dis-
tinguish them  as  tertiary  sexual  characters.
