682 JULIAN S. HUXLEY 
most part only as potentialities of ection. Under severe 
stress (e.g. modern warfare, prolonged worry, &c.), the adult 
system becomes in some way affected. It is no longer so easy 
for the nervous energy to flow along its paths. Under these 
conditions there is more nervous energy available for the other, 
juvenile, system, which has remained undamaged. Finally, 
there will come a moment at which the balance is so altered 
that the adult system ceases te be deminant, and the poten- 
tialitv of the juvenile system is transformed into actuality. 
The juvenile system now becomes dominant in its turn, and 
the adult system retreats into potentiality. During recovery 
a remarkable picture is presented: the two systems are almost 
equally balanced, and we get—not a blending of the effects of 
both—but a rapid alternation, first one and then the other, 
the two never co-existing. A somewhat similar state of affairs 
exists in Perophora ; once absorption of either portion has 
started it proceeds rapidly. Alternation, however, is not 
possible, since in Perophora it is structure, and not merely 
possibilty of function, that is bemg destroyed. 
In the neurological cases structure is not destroyed. Turther, 
the rapidity of change from the dominance of one system to that 
of the other is enormously more rapid, since this is apparently 
accomplished simply by the passing of a threshold-value. 
Once this is passed a sluice is opened, and a different neural 
system flooded so as to permit of function. For this sudden 
appearance of one or the other sub-system some psycho- 
therapeutic writers use the expressive term ‘ puffing-up ’. 
It is a well-known phenomenon of convalescence in such cases. 
Such occurrences are one aspect of the general principle 
laid down by Hughlings Jackson, that, as the result of lesion, 
‘ dissolution occurs first in the most highly-organized products 
of neural or mental activity, leaving the more lowly at liberty 
to express themselves freely in the resulting symptoms’. ‘This, 
however, only stresses the aspect of differential inhibition, not 
that, of equal importance, of intra-organismal struggle. 
Part of this latter aspect of the question is expressed, how- 
ever, by Head (1918), who lays down as one of his general 
principles of neurology that ‘ Integration of function within 
