 The theory of evolution by natural selection suggests that there will be differences as well as similarities across species. Just as Darwin’s finches have different beak shapes depending on the food available on their island, psychological difference should be expected given different environmental pressures. Thus, a Darwinian approach to animal cognition will emphasize differences as well as similarities. Problems arise when the focus is only on questions about similarities without also attending to questions about differences, and vice versa, as we will see in  section 2.