The discussion about the distinction between mood and emotion distracts from the more interesting issue of the evolutionary functions of mood, however, I will attempt to defend my position just a bit more.
2. In psychiatry, the word "mood" is routinely used to denote states of happiness and sadness and their extremes. The Diagnostic and Statistical Manual of the American Psychiatric Association has a major category -- "Mood Disorders" -- which refers specifically to depression and mania. Furthermore, one of my favorite books, "Mood Disorders," is about mania and depression and not other kinds of mood (Whybrow, Akiskal, and McKinney; Plenum Press, 1984). I took pains to specify early in my brief article that I was using mood in this accepted but specialized sense, so there should have been no confusion regarding the object of explanation. Nonetheless, I am sympathetic to the divergent sensitivities of different disciplines. In fact, an earlier draft of the article was originally entitled, "The Evolutionary Origins of the Capacity for Happiness and Sadness." When I found that I had used the phrase "happiness and sadness" 57 times in 6 pages I couldn't stand the verbiage and decided to use the word "mood" instead. Most readers were, I am sure, unknowingly grateful.
3. I agree with Morris that this issue will not be settled by citing authorities on one side or another, but his quote from page 49 of Lazarus (1991) made me worry that I might have misrepresented the book. I was relieved to find, on page 48: "It has been traditional in discussions of emotion to make a distinction between acute emotions and moods. Both are types of emotions. Although the `feel' of an acute emotion and that of a mood are quite different, there are a number of difficulties in making a viable distinction between them." Lazarus then goes on to enumerate the difficulties. He can hardly be said to support "a clear and conventional distinction between mood and emotion."
4. Finally, Morris notes that despite our differing perspectives on the distinction between mood and emotion, we agree substantially on the evolutionary functions of mood. It is quite remarkable how may people have considered the possibility that mood (in the narrow psychiatric sense) regulates activity as some function of resources and the effectiveness of current strategies. I am sorry I did not know about Morris's work on this topic, and look forward to reading it.
American Psychiatric Association (1987). Diagnostic and Statistical Manual of Mental Disorders, third edition, revised. Washington, D.C.: American Psychiatric Association.
Lazarus, R. S. (1991). Emotion and adaptation. New York: Oxford University Press.
Whybrow, P. C.,Akiskal, H. S., & McKinney, W. T. Jr. (1984). Mood Disorders: Toward a New Psychobiology. New York: Plenum.