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who first argued the questions of homo-bio-genesis, a-bio-genesis, and hetero-bio-genesis, had the remotest conception that these questions bore any relation to the spread of small-pox and the influence of vaccination, or were directly involved in the development of anthrax and splenic fever. The inquiry began as a matter of purely physiological research. What was the life history of a bacillus? What were its parentage, development, and offspring? What were its relations to other beings, and to inert matter?
The history of this investigation affords a remarkable illustration of the manner in which the observation of facts, guided by a scientific aim, and prompted by a keen perception of the bearing of scientific truth, has led on by slow degrees to the development of a view of life consistent with itself, and closely interwoven with all that was previously known.
No great discovery has marked any step of its progress, no startling hypothesis has overturned the conceptions of a past age; if we exclude the possibility of its being hereafter proved that each specific disease has its own parasite. The descent from the examination of the higher to that of the lower stratum of life has followed naturally on the improvement of the optical instruments now in familiar use. The germ-theory of life is only the application to the lower classes of what was alredy
