GREAT STEPS IN EVOLUTION 105 



Again, parasitism may pass on one side 

 towards more and more complete mutual 

 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, nor their peculi- 

 arities as unique. These become intelligible 

 products of evolution when we realize them 

 as perhaps the extreme cases of the deter- 

 mination of organism by environment. From 

 the analysis of this relation, especially in 

 these extreme cases of parasite and host, the 

 theory of evolution might almost have been 

 predicted, since, if the details of environ- 

 ment and of organism be, as here, obviously 

 and precisely adapted one to the other, 

 change in the former must either be followed 

 by the extinction of the latter, or by its 

 modification in the requisite details. To 

 understand the modus operandi of this, 

 Weismann invokes the needful germinal 

 variation of the germ-cells, and Dohrn his 

 "principle of functional change" his re- 

 minder that every living tissue, however 



