670 JULIAN S. HUXLEY 
rates, in two different regions, then, if the food-supply is 
reduced, the one which, ceteris paribus, has the higher 
speed will be able to maintain itself in its normal state and at 
its normal level! This may be due to the fact that the 
assimilatory processes are reversible ; this would imply that 
not merely the dissolved food-substances in the body-fluid 
are to be regarded as the ‘ substrate’ from which the various 
reactions draw their materials, but that this substrate must be 
taken as including the tissues themselves. Jf, therefore, two 
reversible reactions A and B were proceeding simultaneously 
in two regions of an organism while the organism was starved, 
we should have each reaction making demands upon the end- 
products of the other, i.e. upon the tissues of the two regions. 
Four processes would therefore be involved—first and secondly, 
the reactions A and B proceeding in their normal direction ; 
thirdly, B proceeding in reversed direction in response to the 
demands of A; and fourthly, A proceeding in reverse direction 
in response to the demands of B. Since A’s speed is greater 
than b’s, the end-product of A will continue to increase, 
while that of B progressively diminishes. We can represent 
such a state of affairs symbolically thus: P =, KX = Q, 
where P is the end-product of A, Q of B, and X the common 
substances utilized by both. If the rate of formation of P is 
greater than that of Q, the reaction will proceed until no Q 
remains.” 
However that may be, we are confronted with the fact that 
if two reaction-systems are competing in the organism for an 
amount of nutriment which is not sufficient for both, then 
the more rapid, or the one which subserves the more highly- 
differentiated region, will not only get first call on the available 
nutriment, but will actually nourish itself at the expense of the 
other. 
1 This again is masked in higher animals by the fact that the nervous 
system, apparently owing to its controlling and co-ordinating function, 
has come to be the system least affected by starvation. 
2 Similar ideas are put forward by Runnstrém (1917) in his important 
paper on dedifferentiation in Echinoid larvae, to which unfortunately 
(owing to the war) I have only just had access, 
