At this point we should include for the sake of completeness reference to a recent theory propounded by Sahli explaining influenza epidemics, a theory to which we do not subscribe. He believes that the pneumococcus, the streptococcus, the influenza bacillus, and possibly other organisms, form a complex group, an obligate complex, a symbiosis, a higher unit, which infects the organism as a unit. It is all of these organisms acting together which produce the influenza. After infection has occurred one or the other member of the group may develop preferentially. In favor of this he says that in one of his cases the sputum was swarming with influenza bacilli on one day, and that the next day the sputum was a thick pure culture of the pneumococcus. He says that if an ultramicroscopic germ should yet be discovered this would not invalidate the theory, but would merely add another member to the group forming the obligate complex virus unit. _Meteorologic conditions._—Formerly attempts were made to demonstrate etiologic relationships between the occurrence of influenza and unusual conditions of the atmosphere. In most cases no relationship has been discovered. Nevertheless it is conceivable that the changes in the atmosphere, particularly seasonal variations, might influence the virulence of the organism. It has been found that nearly all of the many epidemics apparently originating in Russia took their origin there either in the late autumn or in the winter months. The spread of influenza appears to be uninfluenced by atmospheric conditions, but the severity of the disease is definitely increased in the winter months, and Leichtenstern believes that the development of a primary spread from its point of origin is also influenced by the season. Hirsch found that out of 175 correlated pandemics or epidemics, 50 occurred in the winter between December and February, 85 in the spring from March to May, 16 in the summer from June to August and 24 in the autumn. The soil plays no part in the spread of the disease. It prevails on every soil or geologic formation; on the mountain top, in the low malarial swamps, in the tropics and within the arctic circle. Volcanic eruptions, fogs, electrical conditions, ozone, direction of the wind, have all been considered in previous epidemics and successively eliminated as etiologic factors. It must be stated, however, that Teissier, who investigated the influenza in Russia in 1890 and has compared his conclusions at that time with the results of investigation of the recent visitation, believes that some particular cosmic conditions suddenly enhanced the virulence of an endemic etiologic microorganism—probably some ordinary germ—and that this opened the portals to secondary infections.