Natural selection has provided us with a tendency to invest the world with values that it does not contain, demands which it does not make. (p. 135)Desires can motivate, he thinks, but they can also be overcome. I want to do more work, but I can't be bothered, for instance. A sense that something is necessary or must be done, on the other hand, is stronger. Similarly, sympathy might make me do things for others, but if I don't do them, all I will feel is regret. Guilt, on the other hand, is stronger than regret. So conscience, thought of as something like an organ of feelings of a particular type, is a powerful tool.
At first sight this doesn't seem to fit the idea in chapter 5 that action depends on desire, but I suppose pro-attitudes generally could take the place of desires. And a belief that something ought to be done would count as a pro-attitude. Anyone who did not have this belief, though, would still have no reason (as Joyce sees it) to do or not do the act in question. So I might desire to save the baby from the dingo, or I might not feel like it but be motivated nonetheless by my sense that it would be wrong to let the infant be carried off. But if I lack both the desire and the moral belief then I have no reason (and there is no reason) for me to save the baby. (At least, I think this is what Joyce is saying. But I have a hard time thinking he can really mean this. And I lack the motivation to go back and read these chapters again.)
Babies who grow up surrounded by rescuers-from-dingoes will be more likely to survive and reproduce than babies who grow up surrounded by dingoes and shoulder-shruggers, and the former class of babies are likely to have rescuer genes, so natural selection is likely to produce people who like saving others (out of sympathy) or, more likely (because it is more likely to motivate rescues), people who believe (falsely, says Joyce) that they must save babies from dingoes. And keep their promises, tell the truth, etc.