But the possibility of smooth variation seems to be important enough to the effectiveness of natural selection that it is extremely common in actual biological systems. And indeed, while there are some traits—such as eye color and blood type in humans—that are more or less discrete, the vast majority of traits seen, say, in the breeding of plants and animals, show quite smooth variation.

So to what extent does the actual history of biological evolution reflect the kinds of simple characteristics that I have argued one should expect from natural selection?

If one looks at species that exist today, and at the fossil record of past species, then one of the most striking features is just how much is in common across vast ranges of different organisms. The basic body plans for animals, for example, have been almost the same for hundreds of millions of years, and many organs and developmental pathways are probably even still older.

In fact, the vast majority of structurally important features seem to have changed only quite slowly and gradually in the course of evolution—just as one would expect from a process of natural selection that is based on smooth variations in fairly simple properties.

But despite this it is still clear that there is considerable diversity, at least at the level of visual appearance, in the actual forms of biological organisms that occur. So how then does such diversity arise?

One effect, to be discussed at greater length in the next section, is essentially just a matter of geometry. If the relative rates of growth of different parts of an organism change even slightly, then it turns out that this can sometimes have dramatic consequences for the overall shape of the organism, as well as for its mechanical operation.

And what this means is that just by making gradual changes in quantities such as relative rates of growth, natural selection can succeed in producing organisms that at least in some respects look very different.

But what about other differences between organisms? To what extent are all of them systematically determined by natural selection?

Following the discussion earlier in this section, it is my strong suspicion that at least many of the visually most striking differences—