NATURAL SCIENCE IN THE NINETEENTH CENTURY 381 



1879, more fully worked out in 1884 ; the bacillus of tuberculosis 

 in 1882 ; the vibrio of Asiatic cholera in 1883 ; the bacilli of lock- 

 jaw and of diphtheria in 1884 ; the bacillus of bubonic plague in 

 1894 ; and about the same time by others of the microorganisms 

 of malaria, sleeping sickness, and several other diseases. 



In some cases such as smallpox and yellow fever no germs have 

 yet been observed ; but this seems at present to be because they 

 are too small to be seen with the microscope or to be held back, 

 as most germs are, by pipe-clay filters. If, nevertheless, we review 

 the list just given of those plagues in which the causative micro- 

 organism was detected and cultivated between 1879 and 1889, we 

 cannot avoid the conclusion that the ninth decade of the nine- 

 teenth century was the most important hitherto in the history of 

 pathology. When we go further, and compare these rich dis- 

 coveries and the fruit they have since borne, in preventive medi- 

 cine, preventive sanitation and preventive hygiene, with our 

 previous ignorance of the nature of disease and of its control, we 

 realize that since that decade the world has possessed not only a 

 new pathology but also a new science. 



Besides bacteriology another science, parasitology, has also 

 become prominent since the decade of the great pathological 

 discoveries. The parasitic character of the mistletoe, the tape- 

 worm, the flea, the louse, the mosquito and other visible pests 

 was long ago evident, but it was only after the discoveries of Pas- 

 teur and Koch and their disciples were fully comprehended that 

 the germ theory of disease was seen to be at bottom a theory of 

 parasitism. Thereupon parasitology assumed a new place and a 

 new significance as a branch of pathology. 



BIOGENESIS versus SPONTANEOUS GENERATION. The ques- 

 tion of the origin and beginnings of life on the earth has always 

 been obscure and perplexing to mankind, and up to the middle of 

 the nineteenth century the account attributed to Moses, the 

 so-called theory of special creation, was still predominant 

 though about to give way to the theory of evolution. A similar 

 obscurity veiled the beginnings of individual life. Omne vivum 

 ex ovo (all life from the egg) was the motto of those who thought 



