DEDIFFERENTIATION IN PEROPHORA 681 
the ‘nodules’ decreased in size, and leaves it open as to 
whether the cells composing them are actually translocated 
into the main limb-bud, or are simply resorbed into the body. 
The former view is less probable on general grounds, and the 
latter is supported by the facts of resorption in Perophora. 
The limitation of physiological dominance by distance has 
already been brought out by Child (1915 a, chap. 5), but is here 
particularly well illustrated. The relation of dominance to 
simple size-difference between two portions of otherwise 
identical tissue has not, however, so far as I am aware, received 
any special attention, but is obviously of considerable theoretical 
importance. Further, in no other case with which I am familiar, 
is the importance of purely quantitative relations so well 
brought out. It is perfectly clear that inhibition and conse- 
quent resorption can take place at any stage of growth of the 
‘nodule’ (regenerating limb-rudiment), and that it is not due 
to anything in the nodule itself, but entirely to its relations 
with a second developing system. 
We now pass to the very different field of neurology and 
psychology. 
In recent years the phenomenon known as mental regression 
has been carefully studied. Patients suffering from this 
return to an earlier stage of mental existence. Grown men 
may show the behaviour and the mental processes of boys of 
ten or five or even yourger. A review of our knowledge of 
this condition is given by Nichol (1920). 
When properly analysed this state of affairs would seem 
definitely to be due to the presence, in individuals affected by 
it, of two competing systems of mental organization, i.e. of 
two possible main channels for the flow of ‘ nervous energy ’. 
(I purposely use this latter somewhat vague but non-committal 
term to emphasize the fact that the existence of competing 
systems and of some form of activity transmissible along their 
paths is all that we need to assume for a preliminary discussion 
of the problem.) In normal conditions the adult system is 
dominant, the main flow of nervous energy is along its paths, 
and the childish system or systems are dormant, existing for the 
