Now with these simple and yet needful preliminaries you will be able to follow me in a careful study of the least, the very lowliest and smallest, of all living things.  It lies on the very verge of our present powers of optical aid, and what we know concerning it will convince you that we are prepared with competent skill to attack the problem of the life-histories of the smallest living forms.  The group to which the subject of our present study belongs is the bacteria.  They are primarily staff-like organisms of extreme minuteness, but may be straight, or bent, or curved, or spiral, or twisted rods.  This entire projection is drawn on glass, with camera lucida, each object being magnified 2,000 diameters, that is to say, 4,000,000 of times in area.  Yet the entire drawing is made upon an area of not quite 3 inches in diameter, and afterward projected here.  The objects therefore are all equally magnified, and their relative sizes may be seen.  The giant of the series is known as Spirillum volutans; and you will see that the representative species given become less and less in size until we reach the smallest of all the definite forms, and known to science as Bacterium termo.

Now within given limits this organism varies in size, but if a fair average be taken its size is such that 50,000,000 laid in order would only fill the hundredth of a cubic inch.  Now the majority of these forms move with rapidity and grace in the fluids they inhabit.  But how?  By what means?  By looking at the largest form of this group, you will see that it is provided with two delicate fibers, one at each end.  Ehrenberg and others strongly suspected their existence, and we were enabled, with more perfect lenses, to demonstrate their presence some twelve years ago.  They are actually the swimming organs of this Spirillum.  The fluid is lashed rhythmically by these fibers, and a spiral movement of the utmost grace results.  Then do the intermediate forms that move also possess these flagella, and does this least form in nature, viz., Bacterium termo, accomplish its bounding and rebounding movements in the same way?  Yes! by a series of resolute efforts, in using a new battery of lenses—­the finest that at that time had ever been put into the hands of man—­I was enabled to show in succession that each motile form of Bacterium up to B. lineola accomplished its movements by fibers or flagella; and that in the act of self-division, constantly taking place, a new fiber was drawn out for each half before separation.

But the point of difficulty was B. termo.  The demonstration of its flagella was a task of difficulty which only patient purpose could conquer.  But by the use of our new lenses, and special illumination we—­my colleague and I—­were enabled to demonstrate clearly a flagellum at each end of this least of living organisms, as you see, and by the rapid lashing of the fluid, alternately or together,