Viruses bridge the gap, with regard to size, between molecules and organisms. Pure tobacco-mosaic virus protein in particular is physically, chemically, biologically and serologically homogeneous. Although the protein particles are large, they possess the properties of molecules, and hence they are to be regarded as molecules. Among the more important characteristics of this virus protein are its ability to reproduce (only in the living cells of the host plant) and to mutate. The reproduction is regarded as auto-catalytic. Crystal -lizability, homogeneity, ability of reproduction, absence of a cell wall, and absence of any data on respiration, individually considered, do not throw doubt on the animate character of the virus; collectively considered, these characteristics and properties of the virus protein indicate an inanimate substance. Because of the presence of nucleic acid in the virus protein, the virus may conceivably be a degenerated organism having the properties of the virus, but being functionally complete only in the presence of the protoplasm of a living host. Reproduction of the virus is considered to be brought about by secondary valence forces causing an alignment of serologically inactive materials common to the hosts of the virus, much as in the process of crystallization. This alignment and subsequent chem. reactions are considered to be catalyzed by the virus protein on being introduced into the host.