GREAT STEPS IN EVOLUTION 105 



Again, parasitism may pa,s r s on 



rpnrp .nntnlpf.p 



adaptation, witness the symbiosis of alga 

 and animal in certain sea-anemones, or 

 the admirable permanence of that co-opera- 

 tion of short-lived alga and transient mould 

 which enables the resultant lichen sometimes 

 to outlive the very tree which bears it. 

 Galls, again, afford many instances of a 

 parasitism which is reaching equilibration. 



Thus in many ways; we must not consider 

 parasites as simply aberrant "^ t^pir peculi- 

 arities as unique. These become intelligible 

 products of evolution jftjien we jr^a 1 i ^ t f fr f m 

 as perhaps the eYJatjfnft pa^ps. of th p Hptpr- 

 jmnfl.tinn of organism by eT^yironment. From 

 the analysis of this relation, especially in 

 these extreme cases of parasite and host, the 

 theory of evolution miffht almost. )iave hefn 

 predicted, since. if _ the details of environ- 

 ment and of organism b^as b er(^ .ob yioiisly 

 and preoisejy n.d^pted one to the other, 



bft 



hy thff ^xtin^tion of tho latter, or hy itri. 

 TTiodificat-i^p^-^"-^ details. To 



understand the modus operandi of this, 

 Weismann invokes the needful germinal 

 variation of the germ-cells, and D.ohrn his 

 "principle of functional change" 

 minder that eyery living 



