358 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



physiological function. We may ask what knowledge was available at 

 that time to warrant such a suggestion, and the answer is that literally 

 nothing was available save the sheer power of the brain of man. At 

 that time man's physical powers of observation were limited to what 

 could be seen with the unaided eye. No microscope was available, for 

 this instrument did not come on the scene until many hundreds of years 

 later. Yet Aristotle, and other philosophers to follow him, made the 

 suggestion of smaller and still smaller living organisms extending to 

 and literally blending with the nonliving world of metals and stones. 



It was not until 1680 when van Leeuwenhoek invented the micro- 

 scope that man first was enabled to see a new and hitherto unknown 

 world of small living organisms. Yet even this remarkable invention, 

 which eventually extended man's vision down to objects about 300 m,a 

 in diameter, did little immediately to stimulate man's interest in this 

 new microbial world. It was almost 200 years later before Pasteur, 

 Koch, Davaine, and others proved experimentally that these microbes, 

 discovered by means of van Leeuwenhoek's microscope and its succes- 

 sors, were actually responsible for certain infectious diseases. These 

 scientists were extremely vigorous and very successful, for disease after 

 disease was shown to result from the activities of these little living 

 organisms, the bacteria. There followed what has come to be known 

 as the "Golden Era of Bacteriology," during which the causal agents 

 of many important infectious diseases were discovered. It was demon- 

 strated that these bacteria could be grown on nonliving media and 

 that they were, in truth, very small living organisms. Thus the new, 

 usually accepted, borderline between the living and the nonliving 

 worlds was pushed down to the range of about 300 m/x, immediately 

 below which there was the void of the unknown. This void, or twi- 

 light zone, of the unknown extended down to the molecules of the 

 chemist, or to about 10 tci/x. Somewhere in this zone a new dimension 

 was being added, either in the form of a specific relationship between 

 two or more molecules or in the form of a unique type of chemical 

 structure within a single giant molecule. Whatever the form of this 

 new dimension, which confers the ability to reproduce or to bring about 

 replication, it is obvious in retrospect that it should have been of great 

 interest to chemists. Yet chemists tended to disregard this important 

 and intensely interesting field for more than 50 years. 



The thinking of the times was such that in 1892, when Iwanowski, a 

 Russian botanist, found that an infectious disease of the tobacco plant, 

 called tobacco mosaic, was caused by something which would pass 

 through a bacteria-proof filter, that is, a filter which would hold back 

 or retain all known living bacteria, he refused to believe his experi- 

 mental results and concluded that his filters were defective. IwanowsM 

 failed to recognize the fact that he had discovered a new type of inf ec- 



