The possibility that the activity may be due to an impurity must always remain, regardless of the material under discussion. However, since there is no reason to believe that such a situation actually prevails in the case of the virus protein, we are unable at the present time, to conclude other than that the high molecular weight protein under discussion is the virus. Now these same tests that indicate that the virus protein is homogeneous may be used to demonstrate that it possesses the ordinary properties of molecules. As a matter of fact, the chemist, after a perusal of the physical and chemical properties of the tobacco mosaic virus protein, has no difficulty whatsoever in coming to the conclusion that despite its huge size, it has all the properties of a molecule and hence is a molecule. In other words, suspicion arose as to the kinship between viruses and the genes, the basis of reproduction in living beings.
Viruses are able to mutate.
This was another evidence. They also increase in quantity during their sojourn in a living body, and reproduce themselves exactly. Some of them coincide, others are similar and some others different. It is very unfortunate that we do not yet know such essential characters according to which we can definitely establish the degree of relationship between genes and viruses. Until that time we have no right to claim that genes are identical with viruses.