and in his evacuations, objects of great minuteness which differed from each other in form and size and in the peculiar motion which some of them possessed.  In the year 1683 he presented to the Royal Society of London a paper describing a certain minute organism which he found in the tartar of his teeth.  After these observations of Loewenhoeck became known to the world they quickly found application in disease, although the author had expressed himself very cautiously in this regard.  The strongest exponent of the view of a living contagion was Plenciz, 1762, a physician of Vienna, basing his belief not only on the demonstration of minute organisms by Loewenhoeck which he was able to verify, but on certain shrewdly conceived theoretical considerations.  He was the first to recognize the specificity of the epidemic diseases, and argued from this that each disease must have a specific cause.  “Just as a certain plant comes from the seed of the same plant and not from any plant at will, so each contagious disease must be propagated from a similar disease and cannot be the result of any other disease.”  Further he says, “It is necessary to assume that during the prevalence of an epidemic the contagious material undergoes an enormous increase, and this is compatible only with the assumption that it is a living substance.”  But as is so often the case, speculation ran far ahead of the observations on which it is based.  There was a long gap between the observations of Loewenhoeck and the theories of Plenciz, justified as these have been by present knowledge.  In the spirit of speculation which was dominant in Europe and particularly in Germany in the latter half of the eighteenth and the first half of the nineteenth centuries, hypotheses did not stimulate research, but led to further speculations.  As late as 1820 Ozanam expressed himself as follows:  “Many authors have written concerning the animal nature of the contagion of disease; many have assumed it to be developed from animal substance, and that it is itself animal and possesses the property of life.  I shall not waste time in refuting these absurd hypotheses.”  The theory of a living contagion was too simple, and not sufficiently related to the problems of the universe to serve the medical philosophers.

Knowledge of the minute organisms was slowly accumulating.  The first questions to be determined were as to their nature and origin.  How were they produced?  Did they come from bodies of the same sort according to the general laws governing the production of living things, or did they arise spontaneously? a question which could not be solved by speculation but by experiment.  The first experiments, by Needham, 1745, pointed to the spontaneous origin of the organisms.  He enclosed various substances in carefully sealed watch crystals from which the air was excluded, and found that animalculi appeared in the substance, and argued from this that they developed spontaneously.  In 1769, Spallanzani,