An autocatalytic network is a chemical system that outputs a chemical
that is a catalyst
for the original reaction, or that leads to other reactions which eventually output a catalyst for the original reaction. Organisms are stable autocatalytic networks - enzymes accelerate reactions between other enzymes that create more enzymes, and hence the system stabilises. Many of the ingrediants for these reactions are inorganic
substances which are freely available - carbon dioxide
. When living things act as autocatalytic networks, we call it metabolism
Autocatalytic networks should not be viewed as things - they are persistant phenomena, self-maintaining patterns in space and time. They are not made of the same atoms all the way through their existence - they take in matter and use it to grow and replace other atoms. Making the distinction between a dynamic phenomena such as this and something static is quite important.
Autocatalytic networks consume resources. When competition for resources comes about, survival of the fittest comes into play. It is not enough for a network to be stable, it must be "more stable" than the other networks about it. This is natural selection - the most stable and efficient networks persist.