W. M. STANLEY 



quite sure of himself. This indecision becomes apparent when one 

 considers the mode of reproduction and mutation of viruses, whether 

 viruses are molecules or organisms, or whether some are molecules 

 with others being organisms, or whether viruses represent a new type 

 of structure, hitherto unrecognized and undefined, and whether 

 one should speak of a solution or of a suspension of a given virus. 

 For the moment, because of the lack of precise experimental data, 

 discussion of these questions must remain more or less philosophical 

 in nature. But since some of these questions pertain to the very 

 nature of life itself, and others to fundamental physicochemical prob- 

 lems, it is obvious that they are of great importance. During recent 

 years the chemist, as well as the pathologist, has become aware of 

 the limitations of his tools. It has become obvious that, if the tre- 

 mendous problems posed by the viruses are to be solved, it will be 

 necessary to forge new tools, both material and of the mind, and to 

 carry forth the attack on a united front with a new perspective and with 

 renewed courage and vigor. It is the purpose of this short essay to 

 chart briefly the roads that the chemist and pathologist have already 

 constructed into the field of viruses and to attempt to outline, in general 

 terms, the manner in which these can be continued until they join and 

 provide, through a common effort, a broad highway of fact and in- 

 formation leading from the lowly electron to the lofty heights of man. 

 Viruses range in size from about 10 m^i to about 300 m/x. Cer- 

 tain small viruses, such as alfalfa mosaic virus, are smaller than certain 

 accepted protein molecules, such as the Busycon hemocyanin molecules. 

 On the other hand, certain large viruses, such as vaccine virus, are 

 larger than certain accepted organisms, such as the minimal repro- 

 ductive units of the microorganisms of the pleuropneumonia group. 

 With respect to size, therefore, the viruses overlap with molecules at 

 one extreme and with organisms at the other extreme. Since the 

 discovery of viruses by Iwanowski, a plant pathologist, in 1892, and by 

 Beijerinck, a chemist and botanist, in 1898, many investigations on 

 the sizes of diflferent viruses have been carried out. Ultraviolet-light 

 microscopes and ultrafiltration were used in most of these studies. 

 Following the isolation of essentially pure virus preparations, methods 

 involving ultracentrifugation, diffusion, x-ray diffraction, viscosity, 

 osmotic pressure, and stream double refraction measurements were 

 used with great success. Recently the chemist and pathologist have 



