A. D. HERSHEY 



T2 has acquired during its long evolutionary history (excepting 

 what it has delegated to the bacterium). In a sense this is the 

 kind of demonstration that has been the goal of experimental 

 genetics since its beginning, and one that remains a will-o'-the- 

 wisp (16,55) if it is not provided by T2. Second, these fibers 

 must be linear molecules, or very nearly, in which respect they 

 meet the theoretical requirements of a genetic material (68), 

 as the visible chromosomes of cells do not. Third, they can be 

 observed to multiply in a cell that is phylogenetically remote 

 from their proper origin, as the chromosomes of cells cannot. 

 The experimental meaning of this feature is brought out by the 

 fact that both nucleic acids and proteins of virus and host are 

 distinguishable by qualitative analytical methods. In this last 

 respect, the uniqueness is not a matter of opinion. 



Needless to say, the blendor experiment alone does not quite 

 establish these claims (23). A genetic function of the nonstrip- 

 pable tip of the tail has first to be excluded. In itself this may be 

 a trivial point ; it is not likely that even the staunchest biological 

 holist would insist that the fertilized egg contain a representative 

 molecule of every specific substance the organism is capable of 

 producing. The question becomes important in connection with 

 the thesis that viral lineage (defined somewhat abstractly in 

 terms of viral properties that are invariant with respect to varia- 

 tions in the host) is determined exclusively by a structurally 

 homogeneous class of linearly differentiated molecules. It is 

 interesting also because it can be scrutinized by both genetic 

 and chemical methods. Both tend to exclude the tail-tip as 

 the primordium from which new membrane protein comes. 

 I shall give an example of the genetic evidence first. 



The genetic control of the specificity of attachment of virus 

 to bacterium, which means control of the chemical structure of 

 the tail protein, has been studied in a particularly instructive way 

 by several investigators, most thoroughly by Streisinger (59). 

 T2 and T4 are two closely related viruses differing, among other 

 ways, in their tail proteins. Genetic experiments show that 

 this difference is determined by a single gene. We may ask 



