36 
Tuberculosis. 
the  breeds  of  European  cattle.  One  need  not  take  up  the 
extreme  position  of  maintaining  that  all  individual  cattle  of 
the  same  age  are  equally  susceptible,  for  no  two  animals  are 
exactly  alike  in  respect  of  any  of  their  anatomical  or  physio- 
logical characters  ; nor  is  it  necessary  to  deny  that  whatever 
degree  of  susceptibility  any  animal  possesses  is  likely  to  be 
transmitted  to  its  progeny.  From  the  breeder’s  point  of  view 
these  points  are  devoid  of  practical  importance,  because  the 
highest  degree  of  insusceptibility  possessed  by  any  bovine 
animal  is  inadequate  to  enable  it  to  withstand  ordinary  risks 
of  infection ; and  further,  because  there  is  no  practicable 
method  by  which  an  owner,  with  a view  to  selection  for 
breeding  purposes,  can  ascertain  which  of  his  animals  possess 
powers  of  resistance  above  the  average. 
Finally  in  this  connection  a few  words  are  necessary 
regarding  the  susceptibility  of  different  breeds  of  cattle  to 
tuberculosis.  Assuming  that  susceptibility  varies  from  breed 
to  breed,  the  particular  degree  of  susceptibility  possessed  by 
any  breed  is  of  course  transmitted  by  heredity,  like  all  other 
natural  characters.  But  here  again  the  practically  important 
question  is  whether  animals  of  any  breed  possess  what  may  be 
called  a serviceable  immunity  or  insusceptibility — a power 
of  resistance  which  will  preserve  them  from  infection  in 
circumstances  that  determine  the  infection  of  other  breeds. 
At  first  sight  there  appears  to  be  some  evidence  in  favour  of 
the  view  that  susceptibility  does  vary  considerably  from  breed 
to  breed,  for  in  this  and  most  other  countries  the  incidence  of 
the  disease  is  not  uniform  among  all  the  different  breeds. 
The  Shorthorn  and  Jersey  breeds  in  England,  and  the  Ayrshire 
breed  in  Scotland,  furnish  an  exceptionally  large  proportion 
of  victims,  as  compared  with  Herefords,  Galloways,  or  West 
Highland  cattle.  There  is,  however,  no  proof  that  any  one  of 
the  last-mentioned  breeds  possesses  a serviceable  degree  of 
immunity,  and  there  are  good  reasons  for  thinking  that  the 
slighter  incidence  of  the  disease  among  them  is  ascribable  to 
the  fact  that  the  circumstances  under  which  they  are  usually 
bred,  reared,  and  kept,  are  less  favourable  for  the  spread  of  the 
disease  by  contagion  than  in  the  case  of  the  Shorthorn,  Jersey, 
and  Ayrshire  breeds. 
It  is,  however,  unnecessary  to  discuss  this  question  at  any 
length,  for,  even  if  it  were  demonstrated  that  any  breed  were 
less  susceptible  than  another,  the  fact  would  suggest  no 
remedy  against  the  evil  of  high  susceptibility  in  particular 
breeds.  Even  if  the  Hereford  or  the  Galloway  were 
immune,  it  could  not  be  suggested  that  these  ought  to  be 
used  to  displace  the  Shorthorn  or  to  heighten  its  resistance 
by  crossing. 
