﻿704 
  Mutations 
  

  

  ing-primrose 
  as 
  one 
  unit-stride 
  section 
  in 
  the 
  

   great 
  genealogic 
  tree, 
  this 
  period 
  includes 
  two 
  

   nearly 
  related, 
  but 
  not 
  identical 
  changes. 
  One 
  

   is 
  the 
  production 
  of 
  new 
  specific 
  characters 
  in 
  

   the 
  latent 
  condition, 
  and 
  the 
  other 
  is 
  the 
  bring- 
  

   ing 
  of 
  them 
  to 
  light 
  and 
  putting 
  them 
  into 
  active 
  

   existence. 
  These 
  two 
  main 
  factors 
  are 
  conse- 
  

   quently 
  to 
  be 
  assumed 
  in 
  all 
  hypothetic 
  concep- 
  

   tions 
  of 
  previous 
  mutative 
  periods. 
  

  

  Are 
  all 
  mutations 
  to 
  be 
  considered 
  as 
  limited 
  

   to 
  such 
  periods? 
  Of 
  course 
  not. 
  Stray 
  muta- 
  

   tions 
  may 
  occur 
  as 
  well. 
  Our 
  knowledge 
  con- 
  

   cerning 
  this 
  point 
  is 
  inadequate 
  for 
  any 
  definite 
  

   statement. 
  Swarms 
  of 
  variable 
  species 
  are 
  

   easily 
  recognized, 
  if 
  the 
  remnants 
  are 
  not 
  too 
  

   few. 
  But 
  if 
  only 
  one 
  or 
  two 
  new 
  species 
  have 
  

   survived, 
  how 
  can 
  we 
  tell 
  whether 
  they 
  have 
  

   originated 
  alone 
  or 
  together 
  with 
  others. 
  This 
  

   difficulty 
  is 
  still 
  more 
  pronounced 
  in 
  regard 
  to 
  

   paleontologic 
  facts, 
  as 
  the 
  remains 
  of 
  geologic 
  

   swarms 
  are 
  often 
  found, 
  but 
  the 
  absence 
  of 
  

   numerous 
  mutations 
  can 
  hardly 
  be 
  proved 
  in 
  

   any 
  case. 
  

  

  I 
  have 
  more 
  than 
  once 
  found 
  occasion 
  to 
  lay 
  

   stress 
  on 
  the 
  importance 
  of 
  a 
  distinction 
  be- 
  

   tween 
  progressive 
  and 
  retrograde 
  mutations 
  in 
  

   previous 
  lectures. 
  All 
  improvement 
  is, 
  of 
  

   course, 
  by 
  the 
  first 
  of 
  these 
  modes 
  of 
  evolution, 
  

   but 
  apparent 
  losses 
  of 
  organs 
  or 
  qualities 
  are 
  

  

  