106 
ORGANIC DEPENDENCE AND DISEASE 
limitations of time. But we have little except inference to 
build on in applying* this statement to those early periods 
of geological history in which various types of life were 
establishing themselves and with which we have been es- 
pecially dealing. 
Two reasons restrain us from attempting to ascribe such 
high influence to microorganic disease in the early life 
periods of the earth : 
1. Inadequate records. 
2. Protozoan parasitism functions essentially as a chem- 
ical attack upon the tissue and proteid contents of the cell 
of the host for purposes of nutrition, and results in injury 
to such tissue by exudation of active poisons. In metazoan 
parasitism the injury thus caused, if any, and the adjust- 
ment in any cases, are mechanical and due to the functional 
mechanism of the parasite. 
The essential conception of germ disease is the invasion 
of the cell by another cell, in which the invader strikes right 
at the heart of the host by the attack upon the molecular 
constituents of the protein, inclusive of the chromosomes 
which are the recognized carriers of the traits which we 
characterize as individuality and heredity. It is, in other 
words, the attack of the individual cell on the individual cell, 
in which the invaded cell must rouse itself to its full power 
of opposition by the development of greater molecular 
strength and more resistant cell tissue. 
Of such procedures we can learn only from the living 
world. 
The conception that the progress of life by evolution is 
the path which germ disease has permitted and which might 
have been otherwise if not prevented, would seem to tear to 
tatters the obvious plans and purposes laid down in the 
scheme of nature — like a carpet that has been so eaten by 
moths that the pattern is gone and with it the purpose or 
the thought of its maker. We therefore emphasize the dis- 
tinction between protozoan and metazoan parasitism and 
