Page "Adaptation" Paragraph 77

To say something has a function is to say something about what it does for the organism, obviously.

It also says something about its history: how it has come about.

A heart pumps blood: that is its function.

It also emits sound, which is just an ancillary side-effect.

That is not its function.

The heart has a history ( which may be well or poorly understood ), and that history is about how natural selection formed and maintained the heart as a pump.

Every aspect of an organism that has a function has a history.

Now, an adaptation must have a functional history: therefore we expect it must have undergone selection caused by relative survival in its habitat.

It would be quite wrong to use the word adaptation about a trait which arose as a by-product.