GREAT STEPS IN EVOLUTION 107 



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 environment 

 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 reminder that every living tissue, 

 however specialized, retains traces of all the 

 functions of protoplasm, and that there- 

 fore any one of these may be indefinitely 

 increased by favourable conditions, and the 

 specialized function similarly reduced to a 

 trace. Our notion of specialization becomes 

 thus associated with a corresponding possi- 

 bility of simplification, and our idea of 

 progress thus becomes complemented and 

 checked by the possibility of degeneration, and 

 this from any stage of the ascent of life. The 



