In  insects  we  have  the  most  wonderful  illustration  of
the  tearing  down  or  destruction  of  one  structure  and  with
the  same  material  rebuilding  it  into  a  new  and  different
structure. For  instance,  the  cells  first  build  a  caterpillar
or  worm ;  this  worm  as  a  structure  is  demolished  by  those
same  cells  who  put  it  up  and  a  new  and  different  struc-
ture is  put  up  with  the  same  material,  known  as  a  moth
or  butterfly. In  some  other  cases,  if  the  worm  is  cut  in
two,  each  half  will  be  torn  down  by  its  occupants  and  the
half  will  again  be  rebuilt  into  a  complete  worm,  but  each
worm  will  be  only  half  the  size  by  reason  of  the  lack  of
material. There  is  nothing  mysterious  about  this  because
the  structure  is  of  no  use  as  it  is,  so  the  only  thing  to  do
is  to  tear  it  down  entirely  and  rebuild  it. Man  would  do
the  same  thing  if  someone  cut  away  half  of  our  house  and
we  had  no  other  material  to  fix  it  with,  the  only  thing  to
do  would  be  to  tear  it  down  entirely  and  with  the  mate-
rial build  a  smaller  one. Wilson  has  the  following  to  say
on  this  subject :

"Morgan's  remarkable  observation  on  'planaria'  finally
shows  that  here  also  when  the  animal  is  cut  into  pieces,
complete  animals  are  produced  from  these  pieces  but


CAUSE   OF   HEREDITY  291

only  in  small  degrees  through  the  formation  of  new  tis-
sues and  mainly  by  direct  remolding  of  the  old  material
into  a  new  body,  having  the  correct  proportion  of  the
species."

Coming  back  to  the  question  of  variation  I  quote  the
following  from  Mr. Walker:  "Variability  appears  to  be
a  property  common  to  all  living  organisms  in  spite  of  the
fact  that  individual  animals  and  plants  produce  new  in-
dividuals that  are  generally  similar  to  themselves. For
instance,  if  a  collie  dog  be  mated  with  another  collie,  the
pups  produced  will  grow  to  about  the  same  size  as  their
parent,  they  will  have  similarly  shaped  heads  and  be
similar  in  general. Every  individual  in  the  litter  of  pups
will  differ  in  some  way  from  its  brothers  and  sisters  and
also  from  its  parents. But  though  these  differences  are
very  evident,  upon  careful  examination  they  are  in  the
overwhelming  majority  of  instances  comparatively  small
differences. 