Pedigree  Seed  Com. 
125 
how  considerable  and  sometimes  rapid  effects  on  the  racial 
character  of  aggregates  may  be  produced. 
Let  it  be  assumed  that  one  only  out  of  four  seeds  sown 
reproduces  grain.  This  will  be  the  case  with  barley  if  the 
crop  of  grain  is  twelve  and  a half  times  the  quantity  of  seed 
sown,  and  if  the  average  number  of  grains  on  each  plant 
producing  grain  is  fifty  ; a sufficiently  near  approximation 
to  average  field  results. 
Let  it  be  further  assumed  by  way  of  example  that  there 
is  sown  a mixture  composed  of  25  per  cent,  each  of  four 
distinct  races,  A,  B,  C,  D,  and  that  the  resulting  crop  shows  the 
above  ratio  of  crop  to  seed,  viz.,  12'5  : 1. 
The  four  races  will  certainly  differ  in  cropping  capacity. 
If  planted  separately  it  is  probable  that  the  extreme  difference 
in  weight  of  grain  on  equal  areas  would  not  be  more  than 
10  per  cent.,  but  with  the  very  severe  competition  involved  in 
the  obliteration  of  three  out  of  four  seeds  or  plants  at  some 
stage  between  sowing  and  harvesting,  and  taking  into  account 
the  inevitable  differences  in  adaptation  of  the  several  races  to 
the  conditions  of  soil,  climate,  and  cultivation,  it  might  well 
happen  that  the  ratios  of  grain  harvested  to  seed  sown  might 
be  A,  5 : 1,  B,  10  : 1,  C,  15  : 1,  D,  20  : 1,  giving  12‘5  : 1 as 
an  average.  If  the  average  number  of  grains  produced  by 
each  surviving  plant  is  fifty,  a ratio  of  even  20  : 1 for  the  most 
vigorous  race  means  that  more  than  half  the  seeds  or  plants  of 
even  this  race  have  failed  at  some  stage. 
If  seed  is  taken  from  the  resulting  aggregate  and  resown, 
and  the  process  continues  under  the  same  conditions  and  with 
the  same  ratio  of  reproduction  for  each  race  in  each  year,  a 
simple  calculation  shows  that  in  three  years  A,  instead  of  form- 
ing 25  per  cent,  of  the  crop,  will  be  represented  by  less  than 
1 per  cent.  In  six  years  B also  will  be  reduced  to  below  1 per 
cent,  of  the  whole,  whilst  A may  survive  to  the  extent  of  about 
five  plants  per  acre,  C will  survive  considerably  longer,  but  at 
the  end  of  fifteen  years  the  total  of  A,  B,  and  C together  will 
be  less  than  1 per  cent,  of  the  crop,  A and  B being  then  almost 
obliterated.  The  crop  will  be  practically  a pure  race  of  D. 
Even  if  the  ratios  of  reproduction  of  different  races  are 
only  slightly  different,  the  same  result  will  occur  if  the  ratios 
are  steadily  maintained,  and  sufBcient  time  elapses. 
Suppose,  for  instance,  the  average  ratios  of  grain  produced 
to  seed  sown  to  be  A 12  : 1,  B 13  ; 1,  A and  B forming  equal 
proportions  of  the  aggregate  at  the  start,  and  the  produce  to  be 
sown  year  after  year,  in  twenty  years  the  aggregate  will  be 
five  of  B to  one  of  A.  In  fifty  years  there  would  be  fifty-five 
of  B to  one  of  A,  and  in  a hundred  years  3,000  of  B to  one 
of  A— practically  a pure  race  of  B. 
