38 
ORGANIC DEPENDENCE AND DISEASE 
nal directness and independence of life and must be given 
important weight in the conclusion that life started un- 
perturbed and with the best upward purpose ; and even if 
the evidence is essentially negative it loses no force from 
this fact. 
It would seem then that not until life had got in full swing 
did these organic combinations come into existence, even 
in their simplest commensal expressions. Regarding bac- 
teria and sporozoa we have written on a later page, but 
among the invertebrates even the consociation of the anne- 
lids and the corals, which formed easily and early and has 
endured long under manifestations of various sorts, does 
not seem to have yet appeared with the opening of Ordo- 
vician time. 
Relation of Symbiosis to Parasitism 
We have intimated, and it seems a natural presumption, 
that parasitism, by which is meant an adaptation in which 
one organism has become helplessly dependent on another 
for its existence, is the outcome of the innocent combina- 
tions of symbiosis. One would have little difficulty in be- 
lieving that from such a complicated relation of the worms 
to the corals as shown in the Devonian by Pleurodictyum 
and its associates, which we shall presently describe, a con- 
dition of genuine parasitic dependence might well have re- 
sulted, even though the fact is not actually demonstrated. 
It would seem that we must continue to distinguish an in- 
nocent symbiosis from a dependent symbiosis or parasit- 
ism, but this is based only on our present understanding, 
and a statement that the latter can be independent of the 
former and not a consequence upon it seems so illogical that 
it is really not likely to stand up when the facts are more 
far-reaching. In parasitic symbiosis the host is the resist- 
ing, not the consenting or cooperating partner. 
