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Life, an excitable medium .                   47

result in spatial patterns. The two systems involve quite different
molecules; their similarities are in the relationships between them.

Essentially what produces the patterns is a set of reactions with
the following characteristics. First, there is a positive feedback
effect so that a substance stimulates its own production. In the
case of the BZ mixture, the reaction is the following:

HBrO, + BrO^ + 3^ + 2Ce^ -> 2HBrO, + 2Ce^ + H,0

Bromous acid (HBrOi), when mixed with bromate (Br03 ), acid
(H^) and cerium (Ce^) makes more bromous acid (2HBrOi),
oxidized cerium ions (Ce^) and water. Therefore what happens
in a dish of the reagent is that bromous acid starts to produce
more of itself and the concentration builds up, stimulating the
production of the acid in neighbouring regions of the dish so that
a wave of production spreads out. But then the second essential
process comes into play: HBrOi production is inhibited by another
product, 002. There are two possible results of such positive and
negative feedback. Either the rate of production is just balanced
by the rate of destruction so that the system reaches what is known
as a steady state and the net concentration remains constant; or
production and destruction see-saw out of balance and an oscil-
lation occurs. Most chemical reactions reach a balanced condition,
a steady state in which concentrations do not change even although
the substances involved are being constantly produced and con-
sumed. An example is the concentration of oxygen in the atmos-
phere, which is constant at 21% even though oxygen is constantly
being produced by plants and used up by most types of organism
in their metabolic activities. Rates of production and consumption
balance one another to give this constant value. The BZ reaction,
however, because of the positive feedback of bromous acid on its
own production and the complexity of the reactions involved, does
not reach a steady state, production and destruction remaining
out of balance so that the concentrations of the components
change periodically in time or oscillate. The result is that when
the reagents are in an essentially two-dimensional space such as a
thin layer of solution in a flat petri dish, propagating waves of
bromous acid production are initiated from regions where the

How the leopard got its spots



Figure 2.4 Diagram showing some of the components left out of
Weissman 's scheme and not accounted for by molecular biology, in
particular the organization of the generative field that produces the adult
form with influences coming from DNA and cytoplasmic hereditary
factors, and from the environment.





account of the picture as it now appears (Figure 2.4). Instead of
germ plasm (or its modern version, DNA) as the carrier of
the specific inherited factors from the parent that influence the
formation of particular structures in the offspring, we have what
I shall call 'inherited particulars'. These can be either particular
base sequences of DNA that define the genes, or particular
structures of the parent organism such as the melon-stripe Para-
mecium, that get transmitted. These act on the 'generative field',
the organization of the egg cell or the organism itself that grows
and develops to produce a new individual with characteristics
inherited from the parent(s). Separating these two aspects of an
organism is actually artificial, because in reality the two are part
of a single system: the inherited particulars are part of the organ-
ism, which is a field that is organized in space and in time.


The brilliant spotlight that plays upon the genes as the most
distinctive spot on the Darwinian leopard begins to fade into the
context of the whole organism, which now emerges more clearly
as the real living being. For certain purposes it remains useful to
distinguish those aspects of this single dynamic field that are
involved in transmitting particular influences from those that are
responsible for generating form. This convention is actually used
ill the analysis of dynamic systems, as will become clearer in
Chapter 4 where a distinction is made between genes as parameters
and the generative field as the dynamical equations used to describe
the process of producing the form of the developing organism.