That there are great differences in the microbian flora (including herein minute moulds and fungi as well as bacteria) of different localities, is shown by the great experiment of cheese-making which mankind carries on. Each kind of cheese—Stilton, Cheshire, Dutch, Roquefort, Gruyère, Gorgonzola, etc.—is the result of the combined and successive action on milk of a vast number of microbes; and it is the fact that the combination which produces any given kind of cheese is only found and (unconsciously of the exact nature of what he is doing) brought into activity by man at certain places. You cannot make Cheshire cheese in France nor Gruyère in Cheshire, and so on. It is suggested—and the matter is being pursued by experiment and observation at the present time in France—that possibly amongst the other things which go to make up the qualities of the air which agrees or disagrees with one in any given locality—are the local microbes. This must not be regarded as a conclusion which has been fully worked out—it is an ingenious and promising suggestion made by Metchnikoff as the result of some observations, and will be fully inquired into. The fact which I have mentioned above (p. 242)—that the presence of the microbe _Sarcina_ favours the growth of the cholera-bacillus—indeed, enabling it to grow and flourish in conditions in which it was inert until the _Sarcina_ was “sown” alongside of it—renders it worth investigating the question as to whether there are “local” germs or microbes which in this or that region are abundant and get into man’s food and drink, and so into his intestine, and become established there, helping or antagonising the growth of other microbes already there or subsequently introduced. Thus, to the various considerations in regard to the “air” of a locality, such as rarefaction, moisture, temperature, movement, ozone, and the perfumes and exhalations of pine trees, rosemary, and sweet-smelling grasses, which seem to be those which are the most likely to affect the health of inhabitants and visitors, it is not improbable that we must add the influence of an invisible local flora of microbes. The inquiry is a long and laborious one, but it will be carried through. The microbes, whether in air or water, or on the surfaces of things, can be collected by washing them into warm liquid gelatine. Then the gelatine is poured out on a plate, and hardens as a thin sheet of jelly. This is protected from all further contamination, and, after a few hours, each invisible microbe embedded immovably in the jelly makes itself apparent. It multiplies enormously whilst remaining stuck to one spot, and is no longer invisible, but presents itself to the eye as a little sphere, or disc, of characteristic shape, colour, and quality, consisting of many hundred thousands of crowded microbes produced by the growth and division of the original invisible one. Some dozens or some hundreds of little growing “dots,” and of many various kinds, will thus