2i8 LIFE : OUTLINES OF GENERAL BIOLOGY 



and-mouth disease was capable of producing the characteristic 

 symptoms in another animal after it had been passed through a 

 really fine filter which kept back ordinary bacteria. 



The idea spread that the presence of a filterable virus might 

 afford an explanation of many familiar infectious or contagious 

 dis^.Mses in which it had been found imi:K:)ssible microscopically to 

 detect anv microbe. At present under this heading there must be 

 at least half a hundred diseases, and as instances we may mention 

 measles and scarlatina in man, rabies in dogs, pleuro-pneumonia 

 and rinderjx'st in cattle, chicken plague and silkworm jaundice, 

 and various "mosaic diseases" in plants, as in tomatoes, beans, 

 and sugar com. It is probable that these filterable viruses do not 

 form a homogeneous group, but have some of the physiological 

 diversity of the bacteria themselves. Thus it is probable that some 

 of tlie viruses contain an enzyme (or ferment) which does deadly 

 dissolving work, and continues doing so as long as it has suitable 

 material to work on; and this would not be inconsistent with the 

 generally accepted view that the filterable viruses are infinitesimally 

 minute living organisms, comparable in virulence to some of the 

 disease-bacteria and disease-Protozoa, but smaller and probably 

 simpler even than the former. 



The question rises, however, why it has been concluded that 

 these deadly viruses are or contain organisms at all. Why might 

 they not be non-living enzymes, why not simply poisons? The 

 answer is firstly, that there are no facts known that would suggest 

 the passage of a ferment from one organism to another, either 

 infectiously through water, food, or air, or contagiously by actual 

 contact. No doubt the diffusion of some of the filterable-virus 

 disc^ases remains obscure, as in the lamentable case of the foot- 

 and-mouth plague, where secure facts are still few and far between. 

 Hut then- are other cases less obscure where the analogy is certainly 

 with what occurs in indisputable microbic diseases. 



Secondly, in some viruses, such as that associated with chicken 

 plague, the use of devices like centrifuging and the ultra-microscope 

 has been rewarded by the detection of more or less distinctive 

 corpu.scles. In the case mentioned they have been measured and 

 found to Ix? smaller than man's red blood corpuscles. Thirdly, we 

 have to keep in mind the analogy of certain well-known diseases 

 wliere the infective agent is very uncertain, yet the progress of 

 the malady is associated with the appearance of distinctive cor- 

 puscles in the tissues. Some authorities would give, as examples 

 of this state of affairs, the human diseases of smallpox, hydro- 

 phobia, and sudden infantile paraly.sis (epidemic poliomyelitis). 

 It is quite po.ssible that the microbes of several diseases may pass 

 through an "ultra-microscopic" pha.se. 



Lastlv there are cases where the use of an "ultra-filter" robs 



