HEALTH AND DISEASE 299 



some of them at the extreme limit of visibiHty. They were mere single 

 cells, quivering and swimming about, or congeries of such simple cells, 

 often colourless, and in the detritus of organic matter amongst which they 

 were commonly found, it was by no means easy to say what they might 

 or might not be. 



These "bacteria," or "bacteridia," for they were called by many names, 

 had been known to science since the time of Leeuwenhoek, who in 1683 

 first described one of the larger species, which he had seen by squinting 

 through his rudimentary microscope — a single tiny lens mounted in a 

 strip of brass — at some remains of food scraped from his own teeth. In- 

 numerable observations had been made upon them since that time, and by 

 1838 Ehrenberg distinguished what he took to be sixteen distinct species 

 assignable to four genera. After 1844 the study of the bacteria was facili- 

 tated to some extent by DoUand's oil-immersion lens for the microscope 

 which enabled a magnification of one thousand diameters to be obtained. 

 But until a way was found — with the Abbe condenser of 1870 — of con- 

 centrating an intense beam of light upon the minute objects under obser- 

 vation, the scene in the microscope was very dim and ways were not then 

 known of picking out the bacteria by means of stains. Later, when the use 

 of differential stains revealed that the bacteria had distinct cell-walls, some- 

 what resembling those of other plant cells, and that they multiplied by 

 simple fission, the bacteria were claimed by the botanists as Schizomycetes 

 or fission-fungi. A few of the great naturalists, whose comparative studies 

 led them to believe that the law of life would hold down to its lowest mani- 

 festations, were convinced, as Spallanzani had been in the eighteenth cen- 

 tury, that the swarms of bacteria found in infusions of decaying organic 

 matter had their origin in living spores which drifted in the air. That like 

 all other living things they grew from "seeds" or "eggs." For the rest of 

 the world, however, it seemed very plausible that such minute bodies could 

 well be animated by little sparks of life set at liberty when the stuff of 

 larger organisms died or that the processes of fermentation and putre- 

 faction, then regarded as purely chemical, could originate such trifling 

 living things by the way — and this was "spontaneous generation." By 1861, 

 resort to the ancient theory of spontaneous generation to account for the 

 appearance of parasitic fungi on the crops had been thoroughly discredited, 

 but for the smaller organisms, the bacteria, such notions still held sway, 

 and the idea, put forward from time to time, that there might be as many 

 distinct species of bacteria as there were flowers of the field was regarded 

 as an extravagant pleasantry or a precious piece of nonsense. 



As the microscope was improved, and as botanists paid more and more 

 attention to the smallest things in their weird gardens, the fission-fungi 

 might have been admitted quietly into the vegetable kingdom, in the course 

 of twenty years or so, and most people might have taken their biogenesis 

 for granted, but for one historic circumstance — in 1859 Darwin's Origin of 



