﻿Theories 
  of 
  Evolution 
  29 
  

  

  defined 
  and 
  as 
  pure 
  of 
  type 
  as 
  might 
  be 
  ex- 
  

   pected 
  of 
  any 
  species. 
  

  

  These 
  new 
  species 
  are 
  not 
  produced 
  once 
  or 
  

   in 
  single 
  individuals, 
  but 
  yearly 
  and 
  in 
  large 
  

   numbers. 
  The 
  whole 
  phenomenon 
  conveys 
  the 
  

   idea 
  of 
  a 
  close 
  group 
  of 
  mutations, 
  all 
  belonging 
  

   to 
  one 
  single 
  condition 
  of 
  mutability. 
  Of 
  course 
  

   this 
  mutable 
  state 
  must 
  have 
  had 
  a 
  beginning, 
  

   as 
  it 
  must 
  sometime 
  come 
  to 
  an 
  end. 
  It 
  is 
  to 
  

   be 
  considered 
  as 
  a 
  period 
  within 
  the 
  life-time 
  of 
  

   the 
  species 
  and 
  probably 
  it 
  is 
  only 
  a 
  small 
  part 
  

   of 
  it. 
  

  

  The 
  detailed 
  description 
  of 
  this 
  experiment, 
  

   however, 
  I 
  must 
  delay 
  to 
  a 
  subsequent 
  lecture, 
  

   but 
  I 
  may 
  be 
  allowed 
  to 
  state, 
  that 
  the 
  discov- 
  

   ery 
  of 
  this 
  period 
  of 
  mutability 
  is 
  of 
  a 
  definite 
  

   theoretical 
  importance. 
  One 
  of 
  the 
  greatest 
  ob- 
  

   jections 
  to 
  the 
  Darwinian 
  theory 
  of 
  descent 
  

   arose 
  from 
  the 
  length 
  of 
  time 
  it 
  would 
  require, 
  

   if 
  all 
  evolution 
  was 
  to 
  be 
  explained 
  on 
  the 
  

   theory 
  of 
  slow 
  and 
  nearly 
  invisible 
  changes. 
  

   This 
  difficulty 
  is 
  at 
  once 
  met 
  and 
  fully 
  sur- 
  

   mounted 
  by 
  the 
  hypothesis 
  of 
  periodical 
  but 
  

   sudden 
  and 
  quite 
  noticeable 
  steps. 
  This 
  as- 
  

   sumption 
  requires 
  only 
  a 
  limited 
  number 
  of 
  

   mutative 
  periods, 
  which 
  might 
  well 
  occur 
  within 
  

   the 
  time 
  allowed 
  by 
  physicists 
  and 
  geologists 
  

   for 
  the 
  existence 
  of 
  animal 
  and 
  vegetable 
  life 
  

   on 
  the 
  earth. 
  

  

  