24 BIOLOGY: GENERAL AND MEDICAL 



scanned with improved instruments, and its population, 

 when studied and classified, proved to be a miscellane- 

 ous one with varying structural complexity descending 

 to a group so simple that correct classification seemed 

 impossible. These ultimate beings appeared to be 

 neither animals nor plants, and to have no definite place 

 in the general scheme of living things. They were also 

 innumerable, and their distribution apparently universal. 

 Small wonder that it should have been thought a simple 

 matter that what had so little structure, and was neither 

 animal nor vegetable, could arise spontaneously under 

 appropriate conditions. And, in passing, let it be 

 noted that the appropriate conditions under which they 

 appeared thus to arise were to be found in fermentation, 

 putrefaction, and the discharges from morbid tissues 

 conditions that must recall once more the former beliefs 

 about maggots, etc. 



But there were some who conceived that the relation- 

 ship between these minute entities and the conditions 

 under which they were found were the reverse of those so 

 generally accepted, and that instead of the minute organ- 

 isms being generated through fermentative, putrefactive, 

 and morbid conditions, the organisms initiated those 

 conditions, increased in number as they progressed, and 

 died out as they ceased. 



Plenciz, a Viennese physician, greatly interested in 

 the discoveries of Leeuwenhoek, was one of the first who 

 assumed a causal relation between microorganisms and 

 infectious diseases, and published this view as early as 

 1762. He also believed that decomposition only took 

 place when the decomposable material became coated 

 with a layer of organisms, and could only proceed as 

 they increased and multiplied. 



Needham, in 1749, firmly held to the belief that 

 animalculse generated spontaneously as a result of vegeta- 

 tive changes in the substances in which they were found. 

 He maintained that the bacteria that were seen to appear 

 around a grain of barley kept in a carefully covered 



