4 INTRODUCTION 



were curved, some straight, as is shown in F\ they lay ir- 

 regularly and were interlaced. Since I had previously seen 

 living animalculae of this same kind in water, I endeavored 

 to observe whether there was life in them, but in none did I 

 see the smallest movement that might be taken as a sign of 

 life." Leeuwenhoek's observations were purely objective, 

 and lacked the speculative entirely. Other writers, however, 

 within the next century theorized upon his discoveries and 

 worthy of particular mention is Marcus Antonius Plenciz, 

 a physician of Vienna. Plenciz proposed a germ theory of 

 disease in 1762, and taught the etiological relationship of 

 Leeuwenhoek's animalculae to various diseases. He like- 

 wise insisted on a causal relationship of these minute and 

 unnumbered forms of life to the processes of decomposition. 

 Important discoveries regarding bacteria were made by 

 O. F. Miiller, a distinguished Danish zoologist. Miiller made 

 many important observations in regard to the form and struc- 

 ture of bacteria, and studied several types so closely that they 

 can be placed in one or another of the present form types. 

 He first used such terms as bacillus, spirillum, and vibrio, 

 which are in common use now. He also made an attempt to 

 work out (1786) a classification, and in so doing made an 

 observation which has been appreciated by all later workers 

 in this field; namely, " The difficulties that beset the investi- 

 gation of these microscopic animals are complex ; the sure 

 and definite determination (of species) requires so much time, 

 so much of acumen of eye and judgment, so much persever- 



