﻿458 
  Ever-sporting 
  Varieties 
  

  

  ternal 
  conditions 
  I 
  might 
  point 
  to 
  the 
  genus 
  

   Acacia. 
  As 
  we 
  have 
  seen 
  in 
  a 
  previous 
  

   lecture 
  some 
  of 
  the 
  numerous 
  species 
  of 
  this 
  

   genus 
  bear 
  bi-pinnate 
  leaves, 
  while 
  others 
  

   have 
  only 
  flattened 
  leaf-stalks. 
  According 
  to 
  

   the 
  prevailing 
  systematic 
  conceptions, 
  the 
  

   last 
  must 
  have 
  been 
  derived 
  from 
  the 
  first 
  by 
  

   the 
  loss 
  of 
  the 
  blades 
  and 
  the 
  corresponding 
  

   increase 
  of 
  size 
  and 
  superficial 
  extension 
  of 
  the 
  

   stalk. 
  In 
  proof 
  of 
  this 
  view 
  they 
  exhibit, 
  as 
  we 
  

   have 
  described, 
  the 
  ancestral 
  characters 
  in 
  the 
  

   young 
  plantlets, 
  and 
  this 
  production 
  of 
  bi-pin- 
  

   nate 
  leaves 
  has 
  probably 
  been 
  retained 
  at 
  the 
  pe- 
  

   riod 
  of 
  the 
  corresponding 
  negative 
  mutations, 
  

   because 
  of 
  some 
  distinct, 
  though 
  still 
  unknown 
  

   use. 
  

  

  Summarizing 
  the 
  results 
  of 
  this 
  discussion, 
  

   we 
  may 
  state 
  that 
  useful 
  dimorphism, 
  or 
  dou- 
  

   ble 
  adaptation, 
  is 
  a 
  substitution 
  of 
  characters 
  

   quite 
  analogous 
  to 
  the 
  useless 
  dimorphism 
  of 
  

   cultivated 
  ever-sporting 
  varieties 
  and 
  the 
  stray 
  

   occurrence 
  of 
  hereditary 
  monstrosities. 
  The 
  

   same 
  laws 
  and 
  conditions 
  prevail 
  in 
  both 
  cases. 
  

  

  