Tuberculosis. 
33 
this  way,  but  weighty  reasons  for  regarding  it  as  quite 
impossible,  except  when  the  testicles  or  other  parts  of  the 
generative  apparatus  are  tuberculous. 
The  Hereditary  Transmission  of  Predisposition 
to  Tuberculosis. 
As  previously  explained,  belief  in  the  importance  of 
heredity  in  tuberculosis  dates  from  a time  anterior  to  the 
discovery  of  the  tubercle  bacillus,  the  original  conception 
being  that  not  the  disease  itself  but  a peculiar  tissue  weakness 
was  handed  on  from  parent  to  offspring.  After  the  discovery 
of  the  tubercle  bacillus  it  was  for  a time  surmised  that  the 
organism  itself  was  passed  on,  but  when,  for  the  reasons 
already  stated,  that  view  had  to  be  abandoned,  the  original 
conception  was  revived,  and  at  the  present  day  there  are 
still  many  (including  some  eminent  medical  authorities)  who 
maintain  that  inheritance  of  a special  susceptibility  to  in- 
fection, or  an  exceptionally  low  power  of  resisting  the  tubercle 
bacillus,  is  a factor  of  great  importance  in  the  causation  of 
tuberculosis.  It  is  held  that  it  is  impossible  otherwise  to 
account  for  the  tendency  of  the  disease  to  run  in  families. 
The  soundness  of  that  conclusion  must  now  be  examined. 
In  the  first  place  it  must  be  observed  that  there  does  not 
appear  to  be  any  good  reason  for  maintaining  that  bovine 
tuberculosis  is  a disease  which  is  specially  frequent  in  particular 
families  of  any  breed  of  cattle.  The  disease  is  undoubtedly 
more  frequent  in  certain  breeds  than  in  others,  and  the  question 
whether  this  is  in  any  degree  ascribable  to  difference  of  pre- 
disposition will  presently  be  discussed.  But  within  the  same 
breed  the  disease  does  not  exhibit  any  predilection  for  particular 
families.  One  herd  may  be  more  tuberculous  than  another, 
but  it  is  impossible  to  trace  any  connection  between  the 
incidence  of  the  disease  and  pedigree. 
Moreover,  it  is  impossible  to  pretend  that  the  disease  is 
more  rife  in  what  are  called  the  pedigree  herds  of  any  breed 
than  among  the  commoner  bred  animals.  The  fact  is  that  the 
belief  in  the  inheritance  of  special  susceptibility  in  cattle,  like 
the  belief  in  the  hereditary  transmission  of  the  actual  disease, 
would  probably  never  have  obtained  currency  if  medical  men 
had  not  thought  it  impossible  to  account  for  the  observed  facts 
except  on  the  assumption  that  certain  families  of  human  beings 
are  quite  specially  predisposed  to  consumption. 
The  fact  which  is  held  to  justify  that  belief  in  the  case 
of  the  human  species  is  that  consumption  is  far  commoner 
among  the  children  of  consumptive  parents  than  among  the 
children  of  parents  who  appear  to  be  free  from  that  disease. 
But  since  consumption  is  now  universally  admitted  to  be  a 
VOL.  71.  D 
