668 JULIAN S. HUXLEY 
Secondly, we must realize that success in the struggle, i.e. time 
and degree of development, may depend largely on rate of 
metabolic activity. It is not for a moment suggested that this 
is the only factor at work, nor that it is the most important 
factor (in the higher organisms the relationship of the nervous 
system to the tissues of course masks it to a considerable 
extent), but that it is an important factor. 
Child (19156) has drawn attention to its importance for 
problems of regeneration and asexual reproduction; he finds that 
the most actively-working portion of the organism (or, in higher 
forms, the portion containing the higher centres of the nervous 
system) is net only formed first in regeneration, but exerts 
some sort of controlling effect upon the rest of the organization 
of the body. For instance, once a head is formed in the 
regeneration of a Planarian or an Oligochete the old organs 
are remodelled, some bemg broken down, others built up, 
until what exists stands in normal relation to the new head. 
But if, for some reason or other, a head is not formed (in 
Planaria it can be experimentally prevented from forming), 
then this remodelling does not occur. The production of a new 
pharynx, for instance, in a pharynxless posterior half of 
a Planarian, will not take place unless a head is formed at the 
anterior end. 
However, this controlling effect of the head is only exerted 
up to a certain distance. Once this distance is overpast 
the tissues of the bedy are free to react in the way characteristic 
for them when not under any control, i.e. by the formation 
of a new head. In other words this control or dominance of 
the head or oral end (or apical bud in plants) is what regulates 
the important temporal and spatial relations of asexual 
reproduction. As is to be expected, it varies with external 
circumstances, and Child has performed some pretty experi- 
ments on the experimental control of dominance. 
It would appear, especially from some of his recent work 
upon plants, that this dominance exerts an effect analogous 
to that of the nervous system by means of some form of 
conduction, and that it is not, as might at first be expected, 
