ORGANIC DEPENDENCE AND DISEASE 
37 
Once tlie 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 Character 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 history 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 of 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- 
