ORGANIC DEPENDENCE AND DISEASE 37 



Once the dependent habit is established the capacity for 

 reaction grows weaker; degenerative adaptation creeps 

 still further back in the life of successive generations and 

 the degradation of the adult state becomes more profound. 



Complex Chaeacter of Parasitism 



Symbiotic conditions reckoned in terms of the host are 

 often helpful. There is a world full of benign parasites 

 but they are not haphazard. 



True parasitism as known amongst the existing animals 

 and plants is in most cases exceedingly complicated. More- 

 over, when the infesting parasite requires a series of hosts, 

 a different one for each phase of its development, and when 

 in all its stages it is a soft-bodied creature, we must recog- 

 nize the hopelessness of trying to unravel from the geologic 

 record the histors^ of such complex adjustments and be sat- 

 isfied to take them as they are after human ingenuity has 

 succeeded in deciphering them. The course of such per- 

 fected adjustments in evil living may be interesting knowl- 

 edge, but the cause and origin of them can be deciphered 

 only by the mode which we are following through the his- 

 toric study of the more legible expressions of these associa- 

 tions. And it is altogether probable that such complicated 

 careers, especially such as are best known because of their 

 relation to man, are of quite recent adaptations. 



Beginnings op Symbiosis 



Our analysis of the Cambrian fauna has shown the degree 

 to which it has been affected by dependence. So far, how- 

 ever, as our present acquaintance goes, there is no obvious 

 record of symbiotic or commensal conditions in that fauna ; 

 if they occurred at all, they were conditions rarely re- 

 corded. This is a significant fact in its bearing on the origi- 



