peesident's ADDKESS. 21] 



by their consumption of infected leaves. He found also that 

 worms originally infected retained sufficient vitality to produce 

 a cocoon of good silk, but that the next generation became 

 worthless, the disease being infallibly transmitted and thus 

 deteriorating the offspi'ing. The conclusion was obvioiis ; — the 

 larvse were to be examined and no infected specimens were to 

 be used for breeding purposes. This precaution, together with 

 the substitution of a more natural condition of life by breeding 

 in the open air, has been found sufficient to re -invigorate the 

 stock and to keep it in good condition. No wonder, then, that 

 this splendid achievement of M. Pasteur gave an immense im- 

 petus to the investigation of microbes and of their share in the 

 causation of disease. A new literature devoted to the subject 

 soon sprang up and we became familiarised with many minute 

 organisms found to be co-existent with various kinds of disease ; 

 but it was still uncertain whether those organisms were the real 

 cause of disease or only concomitants of the diseased processes. 

 It was not until the quite modern methods of Bacterium-culti- 

 vation were perfected — chiefly through the laborious researches 

 of Professor Koch — that we were enabled to say "with no 

 shadow of doubt whatever" that Bacteria were really thefons et 

 origo mali — not of a few diseases only, but probably of a very 

 large proportion of the ills which flesh is heir to. 



The minute monads known as Bacteria present a considerable 

 variety of form and size, and there was, up to a quite recent 

 period, considerable doubt as to their place in nature,— whether 

 animal or vegetable. It is, however, now generally admitted 

 that they belong to the vegetable kingdom and to the group 

 Schizomycetes or "fission-fungi." This conclusion has been 

 arrived at chiefly from the character of their reproduction, 

 which closely coincides with that of the fungi. Though so 

 extremely small — varying from about 1-6, 000th to 1-25, 000th 

 of an inch in diameter, their reproduction is extremely rapid, 

 and it has been estimated by Cohn {fide Woodhead) that a single 

 bacterium, if placed under favourable conditions, might in the 

 course of three days produce a mass of similar organisms weigh- 

 ing no less than seven thousand five hundred tons, the number 



