144 SYMBIONTICISM AND THE ORIGIN OF SPECIES 



at all times. In this connection, Dr. I. S. Falk ('25) says: 

 "A number and variety of microorganisms and filterable 

 viruses have been incriminated, but none holds an unequiv- 

 ocal claim to a causative relationship The etiol- 

 ogy of epidemic influenza remains a riddle." Such opin- 

 ions appear to indicate that extrinsic factors are responsible 

 for a modification either in the microorganism or in large 

 numbers of human individuals. The peculiar and rapid 

 migration of the disease from East to West around the 

 world, would also indicate that some extrinsic factor is 

 concerned. A disease organism could hardly travel or 

 migrate at the rate at which this disease spreads. 



In discussing the relations of extrinsic factors to epidemi- 

 ology, Falk says: 



The epidemiologist has been accustomed since the rise of the 

 bacteriological fashion to scoff at the statements in older litera- 

 ture concerning the relations between epidemic prevalences and 

 cosmic and telluric phenomena associated with sunlight, weather, 

 rain, wind, earthquake, ground water-level, etc. The evidences 

 which have been coming out of the studies on the vitamin defi- 

 ciency diseases, notably rickets, in recent years, have begun to 

 teach him that he may stay to pray. If recent findings in the 

 value of heliotherapy, especially in relation to organic and cellular 

 physiology, as is so clearly evidenced in rickets and in surgical 

 tuberculosis, have demonstrated anything, they have shown 

 that sunlight plays no inconsiderable role in the maintenance of 

 the normal functioning of the organism. The far-reaching effects 

 of sunlight are reflected in the physiology of parts remote from 

 the surface tissues. These and other environmental factors may 

 prove to be far more significant in the elucidation of certain prob- 

 lems in pathology than has until recently been suspected. 



It would be futile to attempt to analyze, even in a hypo- 

 thetical way, the factors concerned in the development of 

 symbiotic complexes. We can only indicate the factors 

 that influence bacterial behavior so far as we know them, 



