ORGANIC DEPENDENCE AND DISEASE 109 



of the trypanosomes of the deadly sleeping sickness, must 

 not be too quickly interpreted as evidence of the same infes- 

 tation in Tertiary times. It may be so, of course, but the 

 ease and quick change of adjustment, known to every stu- 

 dent of the lower organisms, must invoke caution in such 

 interpretations. 



In connection with these thoughts we naturally revert to 

 Walcott's important determination of a Micrococcus from 

 the Precambrian rocks — the earliest form of life and the 

 simplest; yet, in the opinion of the most competent, indis- 

 tinguishable from still living forms of bacteria. We touch 

 this delectable morsel of fact with caution — it stands as yet 

 alone and independent, but it opens the door to a new, vast 

 and inviting field of microscopic research among the older 

 rocks, which affords the promise of much light upon the 

 history of bacterial functions. 



SUMMARY AND CONCLUSIONS 



Out of the facts and reflections here assembled it would 

 seem that these conclusions regarding the beginnings and 

 the progress of symbiotic dependence may safely be drawn. 



1. Symbiosis was inaugurated very early in the history 

 of life. It visibly existed in the Ordovician fauna and in- 

 ferentially must have been present in the Cambrian ages. 



2. In these early stages of life its occurrence must have 

 been rare and sporadic, even if we grant that it prevailed 

 in the soft-bodied animals or plants that have left no fos- 

 sil remains. 



3. Such combinations, in their inception, were innocu- 

 ous and took on the primary expressions of commensalism 

 or mutualism; partnerships which were in most instances 

 undoubtedly of mutual helpfulness at the start, whatever 

 the risk to progress. In the most notable of these ancient 

 combinations, in which the highly innervated worms were 



