248 THE PRESENT IMPORTANCE OF SCIENCE 



Despite crude and imperfect microscopes, knowledge of 

 these animalcules grew apace, and during the eighteenth 

 century the more important types were recognized. Their 

 discovery reopened the discussion of spontaneous generation, 

 which, a few years before Leeuwenhoek's first observations, 

 had been discredited in the case of insects and the larger 

 organisms. The conflict was renewed between the opposing 

 forces. During this struggle facts were established which 

 not only aided in the final triumph of the modern theory of 

 biogenesis, but also resulted in extensions of knowledge 

 useful in other directions. 



With the advent of the cell-theory in 1839 and with marked 

 improvements in the microscope, the distinction between 

 multicellular and unicellular organisms was established. 

 Finally, the age-long controversy was closed by: Pasteur, in 

 his studies upon disease and fermentation; Tyndall in his 

 examination of the floating matter of the air; Dallinger and 

 Drysdale, who first observed the complete life-cycle of a 

 protozoon; and a host of others. And here, these "nature 

 searchers," who since the days of Leeuwenhoek had been 

 pressing their forces into the seemingly useless fields that 

 teemed with microscopic life, joined with the men long baffled 

 in their fight for human lives, and gave to medicine the 

 support needed in reaching the vantage ground from which 

 to discover a new horizon-line in the germ-theory of disease. 



For a long time, physicians had known that diseases were 

 catching. "The pestilence that walketh in darkness, " was 

 no idle figure of speech. An analogy between the spread of 

 disease and the spread of living organisms had been pointed 

 out for centuries. But only in the nineteenth century, in the 

 generations of our fathers and grandfathers, did the medical 

 men, aided by the investigators who had ventured into the 

 wider domain of abstract science, show that the germ is so 

 truly the cause of infectious disease that without the micro- 

 croscopic germ the disease does not exist. 5 Since the firm 



5 First-hand contact with the medical discussion of this period may be ob- 



