INTRODUCTION 3 



considered along with the pathogenic bacteria. The terms mi- 

 crobe and micro-organism properly include these as well as the 

 bacteria. There is thus an evident tendency to extend the field 

 of bacteriology so that it becomes microbiology or the science of 

 micro-organisms. There are many reasons why this is desir- 

 able. It is certainly essential that the microbes included among 

 the protozoa and the filterable viruses should receive more atten- 

 tion in the future, both from beginning students and from trained 

 investigators. Until separate instruction in these subjects is pro- 

 vided for medical students, they may perhaps best be studied 

 along with bacteriology. 



Biological Relationships. Since the earliest times, the essen- 

 tial difference between living things and lifeless things, that is, 

 the nature of life, has been an interesting subject of speculation. 

 It was at first assumed as a matter of course that the transition 

 from lifeless to living matter readily took place without the 

 agency of preexisting living matter. This speculative assump- 

 tion is still not without its able supporters. The history of actual 

 observations, however, is one long record of refutation of this 

 assumption wherever the facts have been subjected to accurate 

 observation. The ancient Greeks held that living beings arose 

 spontaneously and even Aristotle (384 B.C.) asserted that ani- 

 mals were sometimes formed in this way. These ideas were dis- 

 proved by more careful observation. A notable experiment was 

 that of Francesco Redi (about 1650) who allowed meat to putrefy 

 in a jar covered with fine wire gauze. The flies attracted by the 

 odor deposited their eggs on the gauze and the maggots were 

 hatched there. The assumption that the maggots arose de novo 

 in putrefying meat was thus disproven. Harvey in 1650 made the 

 famous statement, "Omne animal ex ovo" which was later ex- 

 tended to "Omne vivum ex vivo." 



When Anthony van Leeuwenhoek, the "Father of micro- 

 scopy," discovered, described and figured bacteria in 1683, the 

 assumption of spontaneous generation was at once applied to this 

 group of organisms and, although rendered exceedingly doubtful 



