2 BIOLOGY AND TECHNIQUE 



described living beings too small to be seen with the naked eye. There 

 can be no doubt that the small bodies seen by these men and their many 

 immediate successors were, at least in part, bacteria. And indeed the 

 descriptions and illustrations of several of the earliest workers cor- 

 respond with many of the forms which are well known to us at the 

 present day. 



During the century following the work of these pioneers, the efforts 

 of investigators lay chiefly in the more exact morphological description 

 of some of the forms of unicellular life, already known. Conspicuous 

 among the work of this period is that of Otto Friedrich Miiller. In the 

 generation following Miiller's work, however, a marked advance in the 

 study of these forms was made by Ehrenberg,^ who established a 

 classification which, in some of its cardinal divisions, is retained untU 

 the present day. 



Meanwhile the regularity with which these "animalcula" or "in- 

 fusion animalcula " were demonstrable in tartar from the teeth, in intes- 

 tinal contents, in weU-water, etc., had begun to arouse in the minds of 

 the more advanced physicians of the time a suspicion as to a possible 

 relationship of these minute forms with disease. The conception of 

 "contagion," or transmission of a disease from one human being to 

 another, was, however, even at this time, centuries old. The fact had 

 been recognized by Aristotle, had been reiterated by medieval philos- 

 ophers, and had led, in 1546, to the division of contagious diseases by 

 Fracastor, into those transmitted "per contactum," and those con- 

 veyed indirectly "per fomitem." It was for these mysterious facts of 

 the transmissibility of disease, that clinicians of the eighteenth centurj;-, 

 with remarkable insight, saw an explanation in the microorganisms dis- 

 covered by Leeuwenhoek and his followers. 



In fact, Plenciz of Vienna, writing in 1762, not only expressed 

 a belief in the direct etiological connection between microorganisms 

 and some diseases, but was the first to advance the opinion that each 

 malady had its own specific causal agent, which multiplied enormously 

 in the diseased body. The opinions of this author, if translated into 

 the language of our modern knowledge of the subject, came remark- 

 ably near to the truth, not only as regards etiology and transmission, 

 but also in their suggestion of a specific therapy for each disease. 



The conception of a " contagium vivum " was thus practically es- 

 tablished with the work of Plenciz and many others who followed in 



' " Die Infusionstierchen," etc., Leipzig. 1838. 



