76 
The  Physiology  of  Stock-Breeding. 
gave  birth  to  four  pups  which  were  all  closely  alike.  One  of 
these  unfortunately  died  whilst  still  very  young,  but  the  others 
grew  up  into  obvious  mongrels,  showing  little  resemblance  to 
the  Dandie  Dinmont  dog  from  whom  half  of  the  injected  semen 
was  obtained.  It  is,  of  course,  scarcely  legitimate  to  draw  con- 
clusions from  a single  experiment,  but  the  result  is  suggestive 
of  a selective  action  on  the  part  of  the  ova. 
Experience  has  shown  that  inbred  animals  often  tend  to  be 
infertile,  especially  when  mated  with  individuals  that  are  also 
inbred  and  of  the  same  kind,  and  this  is  probably  due  to  a 
decreased  tendency  on  the  part  of  the  germ  cells  to  conjugate, 
comparable  to  that  which  is  known  to  occur  under  certain  con- 
ditions among  unicellular  organisms  or  protozoa.  Moreover, 
among  inbred  animals  it  is  well  known  that  there  is  often  a 
reduction  in  tbe  general  vitality  as  manifested,  for  example,  by 
a decreased  resistance  to  disease.  It  would  seem  extremely 
probable,  therefore,  that  when  two  inbred  animals  are  mated 
together,  the  sum  of  the  vitality  of  the  ova  and  spermatozoa  may 
be  insufficient  to  admit  of  conjugation,  but  that  if  the  sper- 
matozoa are  produced  by  a more  vigorous  stock,  they  may 
succeed  in  fertilising  the  inbred  ova. 
Experiments  by  Calkins'  and  others  on  the  physiology  of 
reproduction  among  the  lowest  forms  of  animal  life  have  shown 
that  the  deterioration  which  results  from  close  inbreeding  may 
in  some  cases  be  obviated  by  a change  of  food  or  surroundings, 
and  that  by  resorting  to  such  means  strains  which  would  other- 
wise die  out  may  survive  to  reproduce  future  generations.  So 
also  among  the  higher  animals  it  would  seem  as  if  a complete 
change  in  the  environment  may  sometimes  lead  to  as  favour- 
able a result  as  an  infusion  of  entirely  fresh  “ blood.”  Thus 
Allison,  the  special  commissioner  of  the  Sportsman,  writes  as 
follows  about  thoroughbreds  imported  into  Australia  : — “ We 
can  draw  from  these  not  only  strains  of  blood  which  we  have 
lost,  but  horses  and  mares,  boim  again  so  to  speak,  and 
admirably  suited  to  strengthen  and  regulate  our  home  stock.”'' 
Breeders  of  game-fowl  in  the  day  of  the  “ cock-pit  ” were 
very  great  advocates  of  in-breeding.  All  reference  to  their 
methods  speak  of  their  practice  in  this  respect.  They  had  an 
idea,  however,  that  in  order  to  prevent  any  ill  effect  from  this 
consanguineous  mating,  it  was  desirable  to  send  their  cockerels 
from  home  to  be  reared.  It  is  said  that  south  country  and 
midland  breeders  would  send  their  young  birds  to  be 
“ walked  ” in  the  north  of  England.  It  is  true  that  this 
practice  had  a double  effect.  The  cockerel  sent  away  as  a 
’ Calkins  : “ Studies  on  the  Life  History  of  Protozoa,”  Jour,  of  Experimental 
Zoology  and  Biological  Bulletin,  1907. 
^ Allison  ; The  British  Thoroughbred  Horse.  London,  1901. 
