9/16 ULTRASTABILITY IN THE ORGANISM 



system demonstrate (if possible) its ultrastability. Whereas the 

 system demonstrates, after the impulse, its power of returning to 

 the state of equilibrium, it demonstrates, after the change of 

 parameter-value, its power of returning the field (of its main 

 variables) to a stable form. The ultrastable system is thus the 

 appropriate' form for the organism if the disturbances that come 

 to it from the world around fall into two clearly defined classes: 



(1) Frequent (or even continuous) small impulsive disturbances 



to the main variables. 



(2) Occasional changes, of step-function form, to its parameters. 



The ultrastable system is thus not merely a didactic device; 

 it may, in some cases, actually be the optimal mechanism by which 

 an organism can ensure its survival. When the disturbances that 

 threaten the organism have, over many generations, had the bi-modal 

 form just described, we may expect to find that the organism will, 

 under natural selection, have developed a form fairly close to the 

 ultrastable, in that it will have developed two readily distinguishable 

 feedbacks. 



9/15. It is not for a moment suggested that all natural stimuli, 

 disturbances, and problems come to kittens in the tidily dichoto- 

 mous way in which we have brought them to the Homeostat. 

 Neither is it suggested that the real brain can always be viewed as 

 ultrastable, if only we can find the right way of approach. On 

 the contrary, it is only when we scientists are fortunate that we 

 will find that a complex system can be reduced conceptually into 

 manageable subsystems, as the Homeostat is reducible into its 

 continuous part with feedbacks between the needles, and its 

 stepwise varying part around the second feedback. If there are 

 many feedback loops, and there is no convenient way of indi- 

 vidualising them, then simplicity is not to be had, and there is 

 nothing for it but to treat the system as one whole, of high com- 

 plexity. (The subject is discussed from Chapter 11 through the 

 remainder of the book.) 



The control of aim 



9/16. The ultrastable systems discussed so far, though develop- 

 ing a variety of fields, have sought a constant goal. The Homeo- 

 stat sought central positions and the rat sought zero grill-potential. 



131 



