﻿Periodic 
  Mutations 
  701 
  

  

  fan 
  itself, 
  and 
  secondly 
  the 
  combination 
  of 
  suc- 
  

   ceeding 
  fans 
  into 
  a 
  common 
  genealogic 
  tree. 
  

  

  The 
  composition 
  of 
  the 
  fan 
  as 
  a 
  whole 
  in- 
  

   cludes 
  more 
  than 
  is 
  directly 
  indicated 
  by 
  the 
  

   facts 
  concerning 
  the 
  birth 
  of 
  new 
  species. 
  They 
  

   arise 
  in 
  considerable 
  quantities, 
  and 
  each 
  of 
  

   them 
  in 
  large 
  numbers 
  of 
  individuals, 
  either 
  in 
  

   the 
  same 
  or 
  in 
  succeeding 
  years. 
  This 
  multiple 
  

   origin 
  must 
  obviously 
  have 
  the 
  effect 
  of 
  

   strengthening 
  the 
  new 
  types, 
  and 
  of 
  heightening 
  

   their 
  chances 
  in 
  the 
  struggle 
  for 
  life. 
  Arising 
  

   in 
  a 
  single 
  specimen 
  they 
  would 
  have 
  little 
  

   chance 
  of 
  success, 
  since 
  in 
  the 
  field 
  among 
  thou- 
  

   sands 
  of 
  seeds 
  perhaps 
  one 
  only 
  survives 
  and 
  

   attains 
  complete 
  development. 
  Thousands 
  or 
  

   at 
  least 
  hundreds 
  of 
  mutated 
  seeds 
  are 
  thus 
  re- 
  

   quired 
  to 
  produce 
  one 
  mutated 
  individual, 
  and 
  

   then, 
  how 
  small 
  are 
  its 
  chances 
  of 
  surviving! 
  

   The 
  mutations 
  proceed 
  in 
  all 
  directions, 
  as 
  I 
  

   have 
  pointed 
  out 
  in 
  a 
  former 
  lecture. 
  Some 
  are 
  

   useful, 
  others 
  might 
  become 
  so 
  if 
  the 
  circum- 
  

   stances 
  were 
  accidentally 
  changed 
  in 
  definite 
  

   directions, 
  or 
  if 
  a 
  migration 
  from 
  the 
  original 
  

   locality 
  might 
  take 
  place. 
  Many 
  others 
  are 
  

   without 
  any 
  real 
  worth, 
  or 
  even 
  injurious. 
  

   Harmless 
  or 
  even 
  slightlv 
  useless 
  ones 
  have 
  

  

  C_J 
  / 
  

  

  been 
  seen 
  to 
  maintain 
  themselves 
  in 
  the 
  field 
  

   during 
  the 
  seventeen 
  years 
  of 
  my 
  research, 
  as 
  

   proved 
  by 
  Oenothera 
  laevifolia 
  and 
  Oeno- 
  

  

  