Leeuwenhoek 3 



bacteria in 1675, designated them as "animalcules." 

 He recognized differences in their appearance and size 

 as well as in their mode of motion, and noted their 

 presence in sea-water and well-water, as well as in 

 various substances of animal origin. These observa- 

 tions, subsequently supplemented by others, gave rise 

 to much speculation and heated discussion concerning 

 the relations of the animalcules to animal diseases. 

 Much interest was attached to the opinion already 

 entertained by some that disease is due to a living 

 contagium. A still greater interest, however, was at- 

 tached, at this time, to the bacteria in connection with 

 the question of spontaneous generation. 



Spontaneous generation. The belief in spontaneous 

 generation is of .ancient origin. Some of the philosophers 

 of the Middle Ages were firm in this belief. Refer- 

 ence may be found in their writings to the spontaneous 

 generation of mice, worms, maggots, and other forms 

 of animal life. With the advance of knowledge, how- 

 ever, the correctness of their facts and conclusions 

 was questioned, and their statements were, in time, 

 discredited and refuted. 



The discovery of bacteria seemed to bring new sup- 

 port to the theory of spontaneous generation. Need- 

 ham announced in 1749 that he had observed the 

 development of bacteria in beef-broth which had been 

 boiled and kept, after that, in a well-stoppered flask. 

 He reasoned that, since no living beings could with- 

 stand boiling, the appearance of the animalcules in 

 the sterile broth must have been due to spontaneous 

 generation. This apparent demonstration of sponta- 



