Aside from the usual updating of material, the major change in this edition is an extensive re-writing of the chapter on memory and learning to emphasize that genes that are involved in behaviour are not immutable but their expression can be modified by transcription factors. Thus, with respect to learning, that old question about which is more important, nature or nuture, genetics or environment, should be answered with the question, which leg is more important for walking, the left or the right?