D.—ZOOLOGY. 111 
Child’s work consisted in applying methods of physiological analysis 
to this well-known process. He found that before any external sign of 
constriction had appeared, the intact and apparently single individual 
showed a hump in the curve exhibiting the rate of chemical change in its 
tissues, when tested from head to tail. The maximum rate of change 
occurred in the head region, and then fell off gradually to rise again to a 
lower peak before the caudal fall. The site of the second, smaller peak was 
the site of the future constriction, and of the future head of the coming 
daughter. 
From this, and a large number of other experiments, Child concluded 
that the head of the parent exercised a variable degree of dominance over 
the subordinate individual that is represented by its own posterior end. 
External features were no longer the criterion of individuality, but merely 
the final expression of the physiological relation of dominance and 
subordination. The nervous system was but one expression of the embodi- 
ment of the dominant region (the brain) and of the track (nerve-cords) 
along which this region exercised its sway. This sway diminished in 
intensity with the length of the cords or distance from the dominant region, 
and it was this gradation of the influence of the “ head’ on the ‘ body’ 
according to distance that Child expressed as the ‘ metabolic gradient.’ 
When the intensity reaches a certain minimum, those portions of the basal 
region whose potential is rising may assert their own hitherto suppressed 
individuality. They become almost physiologically isolated from the 
dominant region. The further conclusion therefore arose, that what we 
are accustomed to think of as an individual multicellular being becomes, 
when interpreted in the dynamic way, a composite being. The intact 
Planarian is only prevented from displaying its constituencies by the 
dominance of the head, but a number of circumstances may interfere with 
the dominance. As the head by growth of the body becomes removed 
from the tail region, the intensity of its influence wanes. If the conduc- 
tivity of the channel of influence falls, the same result ensues. Again, 
*should the tail become the seat of growth, or assert its independence by 
increase of size or in other ways, then the influence of the head is negatived. 
In all cases the head action is positive and not merely inhibitory. In all 
‘eases the basal action on the head is not positive, but indirect or 
inhibitory. 
There are two special assumptions deliberately made by the author of 
this conception of organic individuality that require emphasis. The 
first concerns the independent nature of the apical region, the second the 
use of the term “ metabolic gradient.’ The assumption with regard to the 
first is that the head or apex expresses the most intense and most intimate 
Ttelation between the organism and its environment localised at one pole. 
_ Here the two are really one, and the head is the expression of this fact as 
_ aphysiological, morphological and historic process. The other assumption 
z is based on the physical basis of life as the seat of chemical changes and 
« 
, 
chemical correlation in which it is impossible to distinguish qualitative 
from quantitative effects, and asserts that controlled alteration in the rate 
_ of change (for example, of oxygen consumption) along definite gradients 
_ is the main ‘ cause’ of that structural and chemical correlation that we 
- tall the base. The head or apical region is thus, in a derivative sense, 
 self-determined. It is the animal at its highest; and as these largely 
