and simple underlying programs. But there still remains the question of whether actual biological organisms really use such programs, or whether somehow they instead use much more complicated programs.

Modern molecular biology should soon be able to isolate the specific programs responsible, say, for the patterns on mollusc shells, and see explicitly how long they are. But there are already indications that these programs are quite short.

For one of the consequences of a program being short is that it has little room for inessential elements. And this means that almost any mutation or change in the program—however small—will tend to have a significant effect on at least the details of patterns it produces.

Sometimes it is hard to tell whether changes in patterns between organisms within a species are truly of genetic origin. But in cases where they appear to be it is common to find that different organisms show a considerable variety of different patterns—supporting the idea that the programs responsible for these patterns are indeed short.

So what about the actual process of biological evolution? How does it pick out which programs to use? As a very simple idealization of biological evolution, one can consider a sequence of cellular automaton programs in which each successive program is obtained from the previous one by a random mutation that adds or modifies a single element.

The pictures on the facing page then show a typical example of what happens with such a setup. If one starts from extremely short programs, the behavior one gets is at first quite simple. But as soon as the underlying programs become even slightly longer, one immediately sees highly complex behavior.

Traditional intuition would suggest that if the programs were to become still longer, the behavior would get ever richer and more complex. But from the discoveries in this book we know that this will not in general be the case: above a fairly low threshold, adding complexity to an underlying program does not fundamentally change the kind of behavior that it can produce.

And from this one concludes that biological systems should in a sense be capable of generating essentially arbitrary complexity by using short programs formed by just a few mutations.