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 
