Tuberculosis. 
35 
household,  it  must  be  observed  that  the  fact  is  explainable 
without  calling  in  the  agency  of  inherited  special  predisposi- 
tion. It  is  a fallacy  to  assume  that  in  such  cases  the  late 
development  of  the  disease  means  a late  infection,  for  there 
is  clear  evidence  to  show  that  infection  in  early  life  may 
not  be  manifested  by  outward  symptoms  until  long  afterwards, 
and,  indeed,  may  never  be  followed  by  actual  illness  at  all. 
The  whole  question  in  connection  with  the  alleged  inheri- 
tance of  a special  predisposition  may  be  summed  up  by  saying 
that  there  is  no  evidence  to  prove  that  such  inheritance  plays 
a part  of  any  importance  in  determining  the  incidence  of 
tuberculosis  in  the  human  species,  and  that  all  the  observed 
facts  can  be  reasonably  accounted  for  without  assuming  that 
susceptibility  to  tuberculous  infection  varies  much  from  family 
to  family. 
This  article  is  directly  concerned  with  bovine  tuberculosis 
only,  and  it  may  therefore  be  thought  that  an  undue  amount 
of  space  has  already  been  devoted  to  the  examination  of  ques- 
tions bearing  on  the  causation  of  the  same  disease  in  man. 
What  has  just  been  said,  however,  finds  its  justification  in  the 
circumstance  that  belief  in  the  inheritance  of  a special  pre- 
disposition, as  a factor  of  importance  in  bovine  tuberculosis, 
is  almost  entirely  founded  on  the  supposed  impossibility  of 
accounting  for  the  observed  facts  in  man  on  any  other 
hypothesis  than  that  the  human  victims  of  the  disease  have 
a more  than  average  susceptibility  to  infection,  and  hand  this 
on  to  their  progeny. 
It  must  be  repeated  that  there  are  no  facts  in  connection  with 
the  bovine  disease  which  compel  one  to  admit  that  susceptibility 
to  tuberculosis  varies  greatly  from  individual  to  individual 
or  from  family  to  family.  Moreover,  when  the  question  is 
examined  from  another  point  of  view,  evidence  can  be  found 
which  indicates  that  inherited  special  susceptibility  is  a negli- 
gible factor  in  the  bovine  disease.  To  assert  that,  in  the  case  of 
cattle,  successful  infection  with  tubercle  bacilli  implies  special 
predisposition,  carries  with  it  the  assumption  that  the  average 
bovine  animal  is  non-susceptible  to  a degree  that  enables  it  to 
resist  natural  risks  of  infection.  What  is  the  evidence  of  this  ? 
The  answer  is  that  there  is  none.  No  breeder  dare  assert  that 
the  animals  of  his  herd,  or  any  particular  individuals  in  it,  are 
proof  against  tuberculous  infection,  or  can  remain  healthy 
when  subjected  to  the  risk  created  by  causing  them  to  live  in 
close  association  with  diseased  subjects.  And  this  obviously 
is  the  practically  important  question  in  connection  with  the 
role  of  inheritance  in  bovine  tuberculosis. 
It  cannot  be  too  strongly  emphasised  that  susceptibility 
or  predisposition  to  tuberculosis  is  a normal  character  of  all 
D 2 
