The  hereditary  transmission  of  any  of  these  three  kinds  of
somatogenic  variations  could  be  accounted  for  theoretically  only
by  the  assumption  that  that  part  of  the  soma  which  had  been
changed  by  external  influences,  could  modify  the  germ-plasm
contained  in  the  germ-cells  of  the  same  individual,  so  that  its
offspring  would,  from  the  germ  onwards,  undergo  similar  varia-
tions to  those  which  had  been  acquired  by  the  action  of  external
influences  on  the  parental  part  in  question.
As  far  as  I  can  see,  there  are  only  two  ways  in  which  such  a
variation  could  conceivably  occur  in  the  germ-plasm  in  con-
sequence of  a  corresponding  somatic  variation.  We  should
either  have  to  assume  the  presence  in  all  parts  of  the  body  of
definite  tracks  along  which  each  somatogenic  variation  might
be  transferred  to  the  germ-cells,  in  the  germ-plasm  of  which  it
would  produce  a  corresponding  change ;  or  else  that  gemmules,
such  as  Darwin  supposed  to  exist,  are  given  off  from  every
somatic  cell  and  are  conveyed  to  the  germ-cells, —  either
through  the  vascular  system,  when  one  exists,  —  or  by  some
other  means,  and  that  they  must  then  penetrate  into  these
cells,  and  become  incorporated  in  their  germ-plasm.  Thus
either  the  presence  of  hypothetical  tracks  along  which  a
modifying,  though  totally  inconceivable,  influence  might  be
transferred  to  the  germ-cells,  or  else  the  discharge  of  material
particles  from  the  modified  organ,  must  take  part  in  the  for-
mation of  the  germ-plasm :  there  is  no  third  way  out  of  the
difficulty.
