PARASITIC ADAPTATION. I03 



life, it seems reasonable to conclude that all of the parasitic 

 groups have been derived from free-living forms, and that, as 

 parasitism became a more fixed habit, such structural changes 

 were in the course of time brought about as would make this 

 mode of life obligatory. Furthermore, by comparison of the 

 different parasitic groups, we find all gradations, from scarcely 

 modified forms which are optional or occasional visitors to 

 their hosts, to those of intense parasitic habit, which show 

 the greatest change of structure and most complicated life 

 history. 



