340 
MendeVs  Laws  of  Heredity. 
These  numerical  relationships  would  result  if  it  be  assumed 
that  the  gametes  (the  egg-cells  and  pollen  grains)  carry  only 
one  of  the  pair  of  differentiating  characters,  provided  that  they 
are  produced  in  approximately  equal  numbers.  Thus  on 
crossing  A and  a we  obtain  a cross-bi'ed  whose  gametes  are  not 
Aa  but  either  A or  a.  On  self-fertilisation  a pollen  grain 
carrying  the  differentiating  character  A may  unite  with  an 
egg-cell  can-ying  the  same  character,  or  the  character  a.  It 
may  therefore  give  rise  to  ,4,4  or  Aa.  Similarly  a pollen  grain 
carrying  the  character  a may  give  rise  to  Aa  or  aa.  If  the 
two  types  of  gametes  occur  in  approximately  equal  numbers 
the  possible  progeny  will  be  ,4,4  + Aa  -|-  Aa  -t-  aa,  or 
A + 2,4a  -I-  a.  But  as  A is  dominant  over  a,  it  appears  to 
he  SA  + a.  Where  there  are  two  pairs  of  differentiating 
characters,  A and  a,  B and  b,  the  possible  combinations  are 
AB,  Ab,  aB,  ab,  2Aab,  2AaB,  2ABb,  2aBb,  AAaBb^  or  as 
A and  B are  dominant  over  a and  ft,  these  appear  to  consist  of 
four  groups  only,  AB,  Ab,  aB,  ab,  in  the  ratio  of  9 : 3 : 3 : 1. 
The  subsequent  generation  shows  their  true  character. 
So  much  evidence  of  the  same  kind  has  accumulated  during 
the  last  year  or  so,  both  from  experiments  with  animals  and 
plants,  that  now  it  can  no  longer  be  doubted  that  this  explana- 
tion is  the  correct  one,  and  we  are  justified  in  our  belief  that 
the  gametes  are  pure  with  respect  to  the  characters  they  carry. 
At  the  same  time  these  experiments  have  brought  to  light 
certain  complications  which  need  not  be  discussed  here.  It 
is  sufficient  to  say  that  they  in  no  way  invalidate  the  above 
proposition. 
The  question  which  concerns  us,  as  agriculturists,  is  the 
value  of  these  jjarticular  discoveries.  Will  they  prove  of  any 
value  in  improving  our  stock  or  crops  ? There  need  be  no 
hesitation  in  saying  that  they  will  prove  of  the  utmost  value, 
and  that  agricultural  science  has  few  more  hopeful  pi’oblems 
before  it  than  the  following  up  of  the  lines  of  research  laid 
down  by  Mendel. 
To  test  the  possibilities  of  the  application  of  Mendel’s 
discoveries  a number  of  experiments  have  been  carried  out  on 
the  Experimental  Farm  of  the  Agricultural  Department  of  the 
Cambridge  University.  The  experiments  on  breeding  wheats 
and  barleys  have  now  reached  a stage  at  which  it  is  possible 
to  see  some  definite  results,  and  to  foresee  the  certainty  of 
valuable  discoveries  in  the  near  future. 
Both  of  these  cereals  are  peculiarly  well  adapted  for  a 
study  of  Mendel’s  laws.  There  are  numerous  races  of  wheat 
which  differ  from  one  another  in  constant  chai-acters,  e.g., 
bearded  and  beardless,  rough  and  smooth,  red  and  white,  lax 
and  dense  ears,  &c.  Similarly  among  the  barleys  we  have 
