The  Physiology  of  Stock-Breeding. 
75 
the  germ  cells  just  as  there  is  among  wild  animals,  and  that  the 
ova  which  degenerate  during  the  process  of  development  are 
such  as  are  not  fitted  to  their  surroundings,  or  do  not  I’eceive  a 
suitable  form  of  nutrition  in  the  ovary.  In  this  way  he  explains 
the  apparent  fact  which  is  based  on  statistics  of  births  for  dogs 
and  other  animals,  that  some  conditions  favour  the  production 
of  males  and  others  of  females.  Heape  suggests  that  sex  is 
already  differentiated  in  the  ova,  and  that  under  certain 
circumstances  a larger  number  of  ova  of  one  sex  become  mature, 
and  that  as  a consequence  there  are  a greater  number  of  offspring 
belonging  to  that  sex.  In  a similar  way  he  would  explain  the 
frequency  of  other  sorts  of  variation.  It  ought  to  be  possible, 
therefore,  in  certain  cases  to  discover  the  conditions  which 
determine  the  production  of  one  kind  of  individual  rather  than 
another,  the  idea  being  that  the  external  conditions  affect  the 
ovarian  metabolism,  and  that  only  those  ova  which  happen  to 
be  fitted  to  the  nutritive  conditions  which  prevail  in  the  ovary 
are  able  to  survive.  If  this  conclusion  is  correct — and  the 
evidence  brought  forward  is  considerable — an  enormous  field 
of  work  is  opened  up. 
There  is  also  evidence  that  the  law  of  selection  operates 
among  spermatozoa.  For,  as  already  mentioned,  only  a very 
small  proportion  of  the  spermatozoa  that  are  ejaculated  are  able 
to  conjugate  with  ova,  and  it  is  prima  facie  improbable  that 
every  spermatozoon  has  an  equal  chance  in  the  struggle  for 
existence.  Moreover,  it  is  well  established  that  whereas  some 
males  may  be  sterile  with  individual  females,  they  may  be 
perfectly  fertile  when  serving  other  females.  For  example,  it 
is  said  that  Dorset  Horn  ewes  are  not  infrequently  barren 
when  served  by  tups  of  their  own  breed,  but  are  capable 
of  becoming  pregnant  when  served  by  a Hampshire  Down 
tup.* 
Some  years  ago  I carried  out  an  experiment  which  probably 
has  some  bearing  on  this  point.  I took  a Dandie  Dinmont 
bitch  and  a Dandie  Dinmont  dog,  which  was  some  relation  of 
the  bitch,  and  also  an  ill-bred  Bull  Terrier  dog.  I obtained 
seminal  fluid  from  each  of  the  two  dogs,  and  examined  samples 
under  the  microscope  to  satisfy  myself  that  the  spermatozoa 
were  alive.  I then  mixed  together  equal  quantities  of  the 
semen  of  the  two  animals,  and  finding  on  further  examination 
of  a sample  of  the  mixture  that  all  the  spermatozoa  were  still 
active,  I injected  the  semen  into  the  vagina  of  the  Dandie 
Dinmont  bitch,  which  was  at  that  time  on  heat.  The  bitch  was 
isolated  until  all  signs  of  oestrus  had  passed  off.  A few  weeks 
later  it  was  evident  that  she  had  conceived,  and  at  full  time  she 
* Heape  : “ Abortion,  Barrenness,  and  Fertility  amongst  Sheep,”  Jour. 
Royal  Agrio.  Soc.,  Vol.  10,  1899. 
