Dr. Hamer has visualized the present state of our knowledge of epidemic influenza in a manner which can scarcely be improved upon. We, therefore, quote him at length: “It seems to me that, during the last thirty years or more, we have been making fairly steady uphill progress along the road which constitutes the boundary between the county of epidemiology and the county of bacteriology, and that we have at length reached, at a height considerably above sea level, the foot of the mountain, on the very top of which lies hidden the secret of an ‘epidemic constitution,’ and now we are face to face with a parting of ways. Straight ahead is the frowning height, its summit in cloud. On the right hand stretches away a fine road skirting the base of the mountain. Along this road we have recently seen Dr. Brownlee whirled away in his new car ‘periodogram.’ We are all hoping to hear more from him, but as he is still insisting upon the primary, if not the exclusive, importance of continuous variation in the virulence of the germ, we have to realize that for the time Dr. Brownlee’s road is going down-hill. On the other hand, on the left, there swerves away, through the territory of the old epidemiology, another fine road, which has been explored more particularly by believers in ‘skiey influences.’ So far as it has been traced this road is as flat as flat can be, but of course there is always the possibility that after a while it will begin to rise, as it skirts round the mountain, and leads to a good vantage point from which to start climbing. At the risk, however, of being laughed at, I venture to bring under notice the very rough and at first sight unpromising ground directly in front of us. Along this can be seen two obscurely marked sheep tracks proceeding at any rate onwards and upwards. One diverges slightly towards the left hand and it has been followed at various times by De Schweinitz and others, naturalists bent on collecting ‘ultravisible viruses;’ the second track, directed rather more to the right, has been explored by Reiner, Müller, Massini, Penfold, and others, workers at the problem of discontinuous variation by ‘mutation.’ As a matter of fact I have reason to believe that two travellers, each of whom follows one of these tracks, will keep in sight of and after a while will find that they are approaching one another, and will ultimately meet at a small and retired upland farm; then after passing some dogs and following the track until clear of all stone walls, they will come right out in open ground on the face of the mountain and can start straight up the steep. But it must here be pointed out that there remains to be considered a fourth method of approach to the mountain, the most direct of all; but that is by aeroplane and is of course only open to those trained in metaphysics and statistical methods. Investigators thus equipped are able to rise in the air, to survey with careful scrutiny the whole of the ground beneath