﻿Theories 
  of 
  Evolution 
  21 
  

  

  some 
  peculiarity 
  being 
  found 
  in 
  one 
  of 
  the 
  

   parents 
  and 
  lacking 
  in 
  the 
  other. 
  The 
  second 
  

   are 
  balanced, 
  as 
  all 
  the 
  characters 
  are 
  present 
  

   in 
  both 
  parents, 
  but 
  are 
  found 
  in 
  a 
  different 
  con- 
  

   dition. 
  Active 
  in 
  one 
  of 
  them, 
  they 
  are 
  con- 
  

   cealed 
  or 
  inactive 
  in 
  the 
  other. 
  Hence 
  pairs 
  of 
  

   contrasting 
  units 
  result, 
  while 
  in 
  unbalanced 
  

   crosses 
  no 
  pairing 
  of 
  the 
  particular 
  character 
  

   under 
  consideration 
  is 
  possible. 
  This 
  leads 
  to 
  

   the 
  principal 
  difference 
  between 
  species 
  and 
  va- 
  

   rieties, 
  and 
  to 
  an 
  experimental 
  method 
  of 
  decid- 
  

   ing 
  between 
  them 
  in 
  difficult 
  and 
  doubtful 
  cases. 
  

  

  Having 
  thus 
  indicated 
  the 
  general 
  outlines 
  of 
  

   the 
  subjects 
  I 
  shall 
  deal 
  with, 
  something 
  now 
  

   may 
  be 
  said 
  as 
  to 
  methods 
  of 
  investigation. 
  

  

  There 
  are 
  two 
  points 
  in 
  which 
  scientific 
  in- 
  

   vestigation 
  differs 
  from 
  ordinary 
  pedigree-cul- 
  

   ture 
  in 
  practice. 
  First 
  the 
  isolation 
  of 
  the 
  

   individuals 
  and 
  the 
  study 
  of 
  individual 
  inheri- 
  

   tance, 
  instead 
  of 
  averages. 
  Next 
  comes 
  the 
  task 
  

   of 
  keeping 
  records. 
  Every 
  individual 
  must 
  be 
  

   entered, 
  its 
  ancestry 
  must 
  be 
  known 
  as 
  com- 
  

   pletely 
  as 
  possible, 
  and 
  all 
  its 
  relations 
  must 
  be 
  

   noted 
  in 
  such 
  a 
  form, 
  that 
  the 
  most 
  complete 
  

   reference 
  is 
  always 
  possible. 
  Mutations 
  may 
  

   come 
  unexpectedly, 
  and 
  when 
  once 
  arisen, 
  their 
  

   parents 
  and 
  grand-parents 
  should 
  be 
  known. 
  

   Records 
  must 
  be 
  available 
  which 
  will 
  allow 
  of 
  

   a 
  most 
  complete 
  knowledge 
  of 
  the 
  whole 
  ances- 
  

  

  