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. Eegarding 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. 



KELATION 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. 



