﻿342 
  Ever-sporting 
  Varieties 
  

  

  the 
  mountain-ash 
  (Sorbus 
  Aucuparia) 
  has 
  six 
  

   pairs 
  of 
  leaflets 
  in 
  addition 
  to 
  the 
  terminal 
  one. 
  

   But 
  this 
  number 
  varies 
  slightly, 
  the 
  weaker 
  

   leaves 
  having 
  less, 
  the 
  stronger 
  more 
  pairs 
  than 
  

   the 
  average. 
  Such 
  however, 
  is 
  not 
  the 
  case 
  

   with 
  ternate 
  leaves, 
  which 
  seem 
  to 
  be 
  quite 
  con- 
  

   stant. 
  Four 
  leaflets 
  occur 
  so 
  very 
  rarely 
  that 
  

   one 
  seems 
  justified 
  in 
  regarding 
  them 
  rather 
  as 
  

   an 
  anomaly 
  than 
  as 
  a 
  fluctuation. 
  And 
  this 
  is 
  

   confirmed 
  by 
  the 
  almost 
  universal 
  absence 
  of 
  

   two-bladed 
  clover-leaves. 
  

  

  Considering 
  the 
  deviation 
  as 
  an 
  anomaly, 
  we 
  

   may 
  look 
  into 
  its 
  nature. 
  Such 
  an 
  inquiry 
  

   shows 
  that 
  the 
  supernumerary 
  leaflets 
  owe 
  their 
  

   origin 
  to 
  a 
  splitting 
  of 
  one 
  or 
  more 
  of 
  the 
  nor- 
  

   mal 
  ones. 
  This 
  splitting 
  is 
  not 
  terminal, 
  as 
  is 
  

   often 
  the 
  case 
  with 
  other 
  species, 
  and 
  as 
  it 
  may 
  

   be 
  seen 
  sometimes 
  in 
  the 
  clover. 
  It 
  is 
  for 
  

   the 
  most 
  part 
  lateral. 
  One 
  of 
  the 
  lateral 
  nerves 
  

   grows 
  out 
  becoming 
  a 
  median 
  nerve 
  of 
  the 
  new 
  

   leaflet. 
  Intermediate 
  steps 
  are 
  not 
  wanting, 
  

   though 
  rare, 
  and 
  they 
  show 
  a 
  gradual 
  separa- 
  

   tion 
  of 
  some 
  lateral 
  part 
  of 
  a 
  leaflet, 
  until 
  this 
  

   division 
  reaches 
  the 
  base 
  and 
  divides 
  the 
  leaflet 
  

   into 
  two 
  almost 
  equal 
  parts. 
  If 
  this 
  splitting 
  

   occurs 
  in 
  one 
  leaflet 
  we 
  get 
  the 
  " 
  four-leaved 
  ' 
  

   clover, 
  if 
  it 
  occurs 
  in 
  two 
  there 
  will 
  be 
  five 
  leaf- 
  

   lets. 
  And 
  if, 
  besides 
  this, 
  the 
  terminal 
  leaflet 
  

   produces 
  a 
  derivative 
  on 
  one 
  or 
  both 
  of 
  its 
  sides, 
  

  

  