Now it seems to me that in this effect of an external electric charge upon the internal mechanism of an organism we have a clue to the origin of these new bacteria. I have already told how the fire-ball, in its explosion, shattered the denitrifying cultures in Wotherspoon’s room; and it seems clear that at the moment of the concussion there must have been a tremendous play of electrical forces about the spot. We know hardly anything with regard to the nature of the electrical fields existing in such things as these fire-balls; and it is quite possible that they may be different from anything of which we have any knowledge among the more usual displays of electrical energy. I believe, then, that it is in the action of the fire-ball that we must seek for an explanation of the change in habit of Wotherspoon’s denitrifying bacteria. Again, I have mentioned my observation of the rapid multiplication of the denitrifying bacteria which I made with Wotherspoon’s microscope on the following day. That also seems to me to have a bearing upon the problem; though I admit quite frankly that my evidence is only that of a layman. It is in every way regrettable that Wotherspoon, having tired of using his room as an exhibit, should have cleared away every trace of the wreckage before any expert examination of it could be made; for in this way the crucial evidence on the point was destroyed. Further, in support of my views, I would point out that the very first known occurrence of _B. diazotans_ was that which had Regent’s Park as its site; and that the first place of attack was in the immediate neighbourhood of Wotherspoon’s house in Cumberland Terrace. This can hardly be disregarded, when it is considered in connection with the other facts which I have mentioned. At this time of day there can be no question that London formed the focus from which _B. diazotans_ spread throughout the world. I have described the ramifications of the great air-services; and it seems to me obvious that the organisms were carried to and fro upon the surface of the globe by the agency of the aeroplanes. The order of attack at various points indicates this very clearly, in my opinion. First came the American and Egyptian outbreaks; then Uganda and South America; and finally, long after the others, Australia showed traces of the devastation. I have checked the possible dates of arrival in these various places, taking into account the relative swiftnesses of the aeroplanes on the different routes; and the results can hardly be gainsaid. Allowing, as one must, a certain latitude for the time of development of the microbe in various spots, there seems little doubt that the dates of the outbreaks fell into the same succession as the times of arrival of the various London air-services. CHAPTER IV Panic