54 Hubert P. Yockey 



rate to a tolerable level. According to this principle, we may expect that errors 

 will continue to accumulate in the genome of a given organism until at some 

 point serious difficulty including death will occur. This will be reflected by 

 some value of H, which we call //^, limited by viabihty. An argument for a 

 lower limit H^^ has been given previously (6). 



Errors will accumulate in the genome but at the same time there is a favorable 

 selection for those members of the ensemble which have low equivocation. 

 This represents a certain reserve capacity to withstand the insults of existence. 

 It may therefore be expected in general that the message entropy of the ensemble 

 of organisms will be described by a probability distribution. This distribution 

 can, perhaps, be calculated from first principles, at least for simple cases, when 

 more is known about the storage and transfer of genetical information. 



Death of an organism is defined in different ways in various fields of biology. 

 Permanent loss of reproductive power is the definition of death usually expressed 

 or implied in bacteriology (7). This is the definition chosen in spite of the fact 

 that there are many inteiTnediate stages between the active living cell and the 

 dead cell. It is known that yeast cells which have lost the power to multiply 

 may still be able to fennent (8). Zelle and Hollaender (7) have recently 

 pointed out that attempts to explain the bactericidal effects of irradiation on 

 the basis of one mechanism are unrealistic. In the case of animals the cessation 

 of metabolism, not the loss of fertility, is the criterion of death. These criteria 

 of death are not really different or antagonistic. Since loss of function is implied 

 by loss of information content any experimentally convenient definition of 

 lethality may be used to suit the problem at hand. The lower end of the distribu- 

 tion in message entropy will therefore be determined by the specificity required 

 by the environment. 



A communications analogy may clarify the notion further. Suppose we 

 have a message, with redundance, which is sent through a communication 

 channel with a small but finite noise level. The message contains instructions 

 to perform some necessary task. A recording is made and the message is sent 

 through again, and so forth. Eventually, depending on the noise level of the 

 channel and the redundance in the message, it will be just barely intelligible. No 

 further recordings can be made without loss of part of the required information 

 content. The ensemble of recordings is analogous to the ensemble of organisms. 

 It will be seen in either case that there is a distribution of information content 

 among the elements of the ensemble. 



Individuality finds a place in the theory developed here in a very natural 

 way. This feature corresponds more to reality (9) than theories which must 

 explain non-uniform response as fluctuations. Besides the experiments of 

 Burdette mentioned above it will suffice to note one other example of biological 

 individuality. 



Consider the experiments of Schott (10, 11), Hetzer (12), Lambert (13), 

 GowEN (14), discussed by Gowen (15), on Salmonella tvphimurium in mice and 

 Salmonella gallinarum in fowl. The host population is exposed to the pathogen 

 and the survivors are chosen for further breeding. The case for mice is typical. 

 The survival ratio improved from 18 per cent to 93 per cent in six generations, 

 but remained nearly constant after that. One hundred per cent survival was 

 not achieved. The survival ratio is characteristic of the ensemble not of the 



