﻿Periodic 
  Mutations 
  711 
  

  

  ation 
  of 
  the 
  secular 
  cooling 
  of 
  the 
  earth, 
  as 
  de- 
  

   duced 
  from 
  the 
  increasing 
  temperature 
  in 
  deep 
  

   mines, 
  he 
  concluded 
  that 
  the 
  entire 
  age 
  of 
  the 
  

   earth 
  must 
  have 
  been 
  more 
  than 
  twenty 
  and 
  less 
  

   than 
  forty 
  millions 
  of 
  years, 
  and 
  probably 
  much 
  

   nearer 
  twenty 
  than 
  forty. 
  His 
  views 
  have 
  been 
  

   much 
  criticised 
  by 
  other 
  physicists, 
  but 
  in 
  the 
  

   main 
  they 
  have 
  gained 
  an 
  ever-increasing 
  sup- 
  

   port 
  in 
  the 
  way 
  of 
  evidence. 
  New 
  mines 
  of 
  

   greater 
  depth 
  have 
  been 
  bored, 
  and 
  their 
  tem- 
  

   peratures 
  have 
  proved 
  that 
  the 
  figures 
  of 
  Lord 
  

   Kelvin 
  are 
  strikingly 
  near 
  the 
  truth. 
  George 
  

   Darwin 
  has 
  calculated 
  that 
  the 
  separation 
  of 
  

   the 
  moon 
  from 
  the 
  earth 
  must 
  have 
  taken 
  place 
  

   some 
  fifty-six 
  millions 
  of 
  years 
  ago. 
  Geikie 
  has 
  

   estimated 
  the 
  existence 
  of 
  the 
  solid 
  crust 
  of 
  the 
  

   earth 
  at 
  the 
  most 
  as 
  a 
  hundred 
  million 
  years. 
  

   The 
  first 
  appearance 
  of 
  the 
  crust 
  must 
  soon 
  have 
  

   been 
  succeeded 
  by 
  the 
  formation 
  of 
  the 
  seas, 
  

   and 
  a 
  long 
  time 
  does 
  not 
  seem 
  to 
  have 
  been 
  re- 
  

   quired 
  to 
  cool 
  the 
  seas 
  to 
  such 
  a 
  degree 
  that 
  life 
  

   became 
  possible. 
  It 
  is 
  very 
  probable 
  that 
  life 
  

   originally 
  commenced 
  in 
  the 
  great 
  seas, 
  and 
  that 
  

   the 
  forms 
  which 
  are 
  now 
  usually 
  included 
  in 
  the 
  

   plankton 
  or 
  floating-life 
  included 
  the 
  very 
  first 
  

   living 
  beings. 
  According 
  to 
  Brooks, 
  life 
  must 
  

   have 
  existed 
  in 
  this 
  floating 
  condition 
  during 
  

   long 
  primeval 
  epochs, 
  and 
  evolved 
  nearly 
  all 
  the 
  

   main 
  branches 
  of 
  the 
  animal 
  and 
  vegetable 
  king- 
  

  

  