the life-histories of the putrefactive organisms, although they are the minutest forms of life.  I have stated that they were the inevitable accompaniments of putrescence and decay.  You learned from a previous illustration the general appearance of the Bacteria; they are the earliest to appear whenever putrefaction shows itself.  In fact the pioneer is this—­the ubiquitous Bacterium termo. The order of succession of the other forms is by no means certain.  But whenever a high stage of decomposition is reached, a group of forms represented by these three will swarm the fluid.  These are the Monads, they are strictly putrefactive organisms, they are midway in size between the least and largest Bacteria, and are, from their form and other conditions, more amenable to research, and twelve years ago I resolved, with the highest power lenses and considerable practice in their use, to attack the problem of their origin; whether as physical products of the not-living, or as the natural progeny of parents.

But you will remember that only a minute drop of fluid containing them can be examined at one time.  This minute drop has to be covered with a minute film of glass not more than the 1/200 of an inch thick.  The highest lenses are employed, working so near as almost to touch the delicate cover.  Clearly, then, the film of fluid would rapidly evaporate and cause the destruction of the object studied.  To prevent this an arrangement was devised by which the lens and the covered fluid under examination were used in an air-tight chamber, the air of which was kept in a saturated condition; so that being, like a saturated sponge, unable to take in any more, it left the film of fluid unaffected.  But to make the work efficient I soon found that there must be a second observer.  Observation by leaps was of no avail.  To be accurate it must be unbroken.  There must be no gap in a chain of demonstration.  A thousand mishaps would occur in trying to follow a single organism through all the changes of successive hours to the end.  But, however many failures, it was evident, we must begin on another form at the earliest point again, and follow it to the close.  I saw soon that every other method would have been merely empirical, a mere piecemeal of imagination and fact.  When one observer’s ability to continue a long observation was exhausted, there must be another at hand to take up the thread and continue it; and thus to the end.  I was fortunate indeed at this time in securing the ready and enthusiastic aid of Dr. J.J.  Drysdale, of Liverpool, who practically lived with me for the purpose, and went side by side with me to the work.  We admitted nothing which we had not both seen, and we succeeded each other consecutively, whenever needful, in following to the end the complete life-histories of six of these remarkable forms.